UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K

OR

For fiscal year ended December 31, 2005

COMMISSION FILE NO. 1-6622

WASHINGTON REAL ESTATE INVESTMENT TRUST

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code (301) 984-9400

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    YES  x    NO  ¨

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    YES  ¨    NO  x

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding twelve (12) months (or such shorter period that the Registrant was required to file such report) and (2) has been subject to such filing requirements for the past ninety (90) days.    YES  x    NO  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of the Registrant’s knowledge in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.  x

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer or a non-accelerated filer (see definition of “accelerated filer and large accelerated filer” in Exchange Act Rule 12b-2).    YES  x    NO  ¨

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Act).    YES  ¨    NO  x

As of February 27, 2006 42,175,428 Shares of Beneficial Interest were outstanding. As of June 30, 2005, the aggregate market value of such shares held by non-affiliates of the registrant was approximately $1,314,238,598 (based on the closing price of the stock on June 30, 2005).

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the Trust’s definitive Proxy Statement relating to the 2006 Annual Meeting of Shareholders, to be filed with the Securities and Exchange Commission, are incorporated by reference in Part III, Items 10-14 of this Annual Report on Form 10-K as indicated herein.

WASHINGTON REAL ESTATE INVESTMENT TRUST

2005 FORM 10-K ANNUAL REPORT

INDEX

PART I

The Trust

Washington Real Estate Investment Trust (“WRIT,” the “Trust,” or the “company”) is a self-administered, self-managed, equity real estate investment trust (“REIT”) successor to a trust organized in 1960. Our business consists of the ownership and development of income-producing real properties in the greater Washington—Baltimore region. We own a diversified portfolio of general purpose office buildings, medical office buildings, industrial/flex properties, multifamily buildings and retail centers.

We believe that we qualify as a REIT under Sections 856-860 of the Internal Revenue Code and intend to continue to qualify as such. To maintain our status as a REIT, we are required to distribute 90% of our ordinary taxable income to our shareholders. When selling properties, we have the option of (i) reinvesting the sale price of properties sold, allowing for a deferral of income taxes on the sale, (ii) paying out capital gains to the shareholders with no tax to the company or (iii) treating the capital gains as having been distributed to the shareholders, paying the tax on the gain deemed distributed and allocating the tax paid as a credit to the shareholders. $33.5 million of gains from property disposed in 2005 was reinvested in replacement properties. Approximately $3.5 of gains from disposed property in 2005 was distributed to share holders. We distributed all of our 2005, 2004 and 2003 ordinary taxable income to our shareholders. Gains from the property disposed in 2004 were distributed to shareholders. No provision for income taxes was necessary in 2005, 2004 or 2003. Over the last five years, dividends paid per share have been $1.60 for 2005, $1.55 for 2004, $1.47 for 2003, $1.39 for 2002 and $1.31 for 2001.

We generally incur short-term floating rate debt in connection with the acquisition of real estate. As market conditions permit, we replace the floating rate debt with fixed-rate secured loans or unsecured senior notes, or repay the debt with the proceeds of sales of equity securities. We may acquire one or more properties in exchange for our equity securities or operating partnership units which are convertible into WRIT shares.

Our geographic focus is based on two principles:

We consider markets to be local if they can be reached from the Washington centered market within two hours by car. Our Washington centered market reaches north to Philadelphia, Pennsylvania and south to Richmond, Virginia. While we have historically focused most of our investments in the greater Washington/Baltimore Region, in order to maximize acquisition opportunities we will and have considered investments within the two-hour radius described above. We will also consider opportunities to duplicate our Washington focused approach in other geographic markets which meet the criteria described above.

All of our Trustees, officers and employees live and work in the greater Washington/Baltimore region and our officers average over 20 years of experience in this region.

This section includes or refers to certain forward-looking statements. You should refer to the explanation of the qualifications and limitations on such forward-looking statements beginning on page 46.

The Greater Washington/Baltimore Economy

2005 proved to be a year of strong performance for the greater Washington/Baltimore area with both the professional and business services sectors fueling job growth. Federal procurement spending continues to be strong, particularly in the defense industry, with its issuance of defense, intelligence and security contracts. Continued spending by the General Services Administration and the corresponding government contracting firms and professional services firms is expected to further drive regional growth. Office leasing activity has increased

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and has had a positive impact on the industrial and multifamily rental markets. Retail leasing space has been positively affected by the Metro area’s overall population growth and high levels of discretionary income.

We believe regional job growth in 2006 will continue to be driven by professional services firms, including government contractors. According to Delta Associates / Transwestern Commercial Services (“Delta”), a national full service real estate firm that provides market research and evaluation services for commercial property types including office, industrial, retail and apartments:

While growth is very important, from an investment perspective, economic stability is equally important. The Federal government, professional/business services and transportation are the core industries in the greater Washington/Baltimore area economy. Increased spending by the Federal government is expected to continue driving regional economic growth. Federal government spending in the region increased 15% in 2005 and accounts for 17% of the Gross Regional Product.

Greater Washington/Baltimore Real Estate Markets

The economic stability in the greater Washington/Baltimore region has translated into stronger relative real estate market performance in each of our four sectors, compared to other national metropolitan regions as reported by Delta:

Office and Medical Office Sectors

Multifamily Sector

Grocery-Anchored Retail Centers Sector

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Industrial/Flex Sector

WRIT PORTFOLIO

As of December 31, 2005, we owned a diversified portfolio of 68 properties consisting of 21 office buildings, 7 medical office buildings, 12 retail centers, 9 multifamily buildings and 19 industrial/flex properties. Our principal objective is to invest in high quality properties in prime locations, then proactively manage, lease, and develop ongoing capital improvement programs to improve their economic performance. The percentage of total real estate rental revenue by property group for 2005, 2004 and 2003 and the percent leased, calculated as the percentage of physical net rentable area leased, as of December 31, 2005 were as follows:

On a combined basis, our portfolio was 94% leased at December 31, 2005 and 92% leased at December 31, 2004 and 2003.

Total rental revenue from continuing operations was $190.0 million for 2005, $171.6 million for 2004 and $153.6 million for 2003. During the three year period ending December 31, 2005, we acquired two office buildings, five medical office buildings, two retail centers and five industrial properties. During that same time frame, we sold three office buildings and one industrial property. These acquisitions and dispositions were the primary reason for the shifting of each group’s percentage of total revenue reflected above.

No single tenant accounted for more than 3.3% of revenue in 2005, 3.3% of revenue in 2004, and 2.5% of revenue in 2003. All Federal government tenants in the aggregate accounted for approximately 1.8% of our 2005 total revenue. Federal government tenants include the Department of Defense, U.S. Patent and Trademark Office, Federal Bureau of Investigation, Office of Personnel Management, U.S. Department of Consumer Affairs and the National Institutes of Health. WRIT’s larger non-Federal government tenants include the World Bank, Sunrise Senior Living, Inc., Lockheed Corporation, George Washington University, IQ Solutions, Sun Microsystems, INOVA Health Systems, United Communications Group and Westat.

We expect to continue investing in additional income producing properties. We only invest in properties which we believe will increase in income and value. Our properties compete for tenants with other properties throughout the respective areas in which they are located on the basis of location, quality and rental rates.

We have recently engaged in ground-up development in order to further strengthen our portfolio with long-term growth prospects. We currently have three ground-up development projects underway. The first is a 224-unit

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mixed-use residential and retail property in Arlington, VA referred to as Rosslyn Towers, with completion of the mid rise building expected in late 2006 and the high rise building expected in the second quarter 2007. The second is a 75-unit mixed-use residential and retail property in Alexandria, VA referred to as South Washington Street, with completion expected in early 2007. The third is our December 2005 acquisition of Dulles Station in Herndon, VA. These two office buildings will total 540,000 square feet when the two phases are complete. The completion of the first 185,000 square feet of building construction is expected late in 2007 and the second building of 355,000 square feet is expected to be completed sometime in 2008 or 2009 depending on market conditions.

We make capital improvements on an ongoing basis to our properties for the purpose of maintaining and increasing their value and income. Major improvements and/or renovations to the properties in 2005, 2004, and 2003 are discussed under the heading “Capital Improvements.”

Further description of the property groups is contained in Item 2, Properties and in Schedule III. Reference is also made to Item 7, Management’s Discussion and Analysis of Financial Condition and Results of Operations.

On February 28, 2006, we had 260 employees including 179 persons engaged in property management functions and 81 persons engaged in corporate, financial, leasing and asset management functions.

AVAILABILITY OF REPORTS

A copy of this Annual Report on Form 10-K, as well as our Quarterly Reports on Form 10-Q, Current Reports on Form 8-K and any amendments to such reports are available, free of charge, on the Internet on our website www.writ.com. All required reports are made available on the website as soon as reasonably practicable after they are electronically filed with or furnished to the Securities and Exchange Commission. The reference to our website address does not constitute incorporation by reference of the information contained in the website and such information should not be considered part of this document.

Set forth below are the risks that we believe are material to our shareholders. We refer to the shares of beneficial interest in Washington Real Estate Investment Trust as our “shares,” and the investors who own shares as our “shareholders.” This section includes or refers to certain forward-looking statements. You should refer to the explanation of the qualifications and limitations on such forward-looking statements beginning on page 46.

Our performance and value are subject to risks associated with our real estate assets and with the real estate industry.

Our economic performance and the value of our real estate assets are subject to the risk that if our office, medical office, industrial, multifamily and retail properties do not generate revenues sufficient to meet our operating expenses, including debt service and capital expenditures, our cash flow and ability to pay distributions to our shareholders will be adversely affected. The following factors, among others, may adversely affect the revenues generated by our commercial and multifamily properties:

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We are dependent upon the economic climate of the greater Washington/Baltimore region.

All of our properties are located in the greater Washington/Baltimore region. General economic conditions and local real estate conditions in this geographic region have a particularly strong effect.

We face risks associated with property acquisitions.

We intend to continue to acquire properties which would continue to increase our size and could alter our capital structure. Our acquisition activities and success may be exposed to the following risks:

We may acquire properties subject to liabilities and without recourse, or with limited recourse, with respect to unknown liabilities. As a result, if liability were asserted against us based upon the acquisition of a property, we may have to pay substantial sums to settle it, which could adversely affect our cash flow. Unknown liabilities with respect to properties acquired might include:

We face new and different risks associated with property development.

The ground-up development of Rosslyn Towers, South Washington Street and Dulles Station, as opposed to renovation and redevelopment of an existing property, is a relatively new activity for WRIT. Developing properties, in addition to the risks historically associated with our business, presents a number of new and additional risks for us, including risks that:

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Properties developed or acquired for development may generate little or no cash flow from the date of acquisition through the date of completion of development. In addition, new development activities, regardless of whether or not they are ultimately successful, may require a substantial portion of management’s time and attention.

We face potential difficulties or delays renewing leases or re-leasing space.

From 2006 through 2010, leases on our Office, Medical Office, Retail and Industrial properties will expire on a total of approximately 69% of our leased square footage as of December 31, 2005, with leases on approximately 14% of our leased square footage expiring in 2006, 11% in 2007, 15% in 2008, 14% in 2009 and 15% in 2010. We derive substantially all of our income from rent received from tenants. Also, when our tenants decide not to renew their leases, we may not be able to re-let the space. If tenants decide to renew their leases, the terms of renewals, including the cost of required improvements or concessions, may be less favorable than current lease terms. As a result, our cash flow could decrease and our ability to make distributions to our shareholders could be adversely affected. Residential properties are leased under operating leases with terms of generally one year or less. For the years ended 2005, 2004 and 2003, the residential tenant retention rate was 57%, 59% and 53%, respectively.

We face potential adverse effects from major tenants’ bankruptcies or insolvencies.

The bankruptcy or insolvency of a major tenant may adversely affect the income produced by a property. Although we have not experienced material losses from tenant bankruptcies or insolvencies in the past, a major tenant could file for bankruptcy protection or become insolvent in the future. We cannot evict a tenant solely because of its bankruptcy. On the other hand, a court might authorize the tenant to reject and terminate its lease. In such case, our claim against the bankrupt tenant for unpaid, future rent would be subject to a statutory cap that might be substantially less than the remaining rent actually owed under the lease, and, our claim for unpaid rent would likely not be paid in full. This shortfall could adversely affect our cash flow and results from operations. If a tenant experiences a downturn in its business or other types of financial distress, it may be unable to make timely rental payments.

Our properties face significant competition.

We face significant competition from developers, owners and operators of office, medical office, industrial, multifamily, retail and other commercial real estate. Substantially all of our properties face competition from similar properties in the same market. Such competition may affect our ability to attract and retain tenants and may reduce the rents we are able to charge. These competing properties may have vacancy rates higher than our properties, which may result in their owners being willing to make space available at lower prices than the space in our properties.

Compliance or failure to comply with the Americans with Disabilities Act and other laws could result in substantial costs.

The Americans with Disabilities Act generally requires that public buildings, including commercial and multifamily properties, be made accessible to disabled persons. Noncompliance could result in imposition of

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fines by the Federal government or the award of damages to private litigants. If, pursuant to the Americans with Disabilities Act, we are required to make substantial alterations and capital expenditures in one or more of our properties, including the removal of access barriers, it could adversely affect our financial condition and results of operations, as well as the amount of cash available for distribution to our shareholders. We may also incur significant costs complying with other regulations. Our properties are subject to various federal, state and local regulatory requirements, such as state and local fire and life safety requirements. If we fail to comply with these requirements, we may incur fines or private damage awards. We believe that our properties are currently in material compliance with all of these regulatory requirements. However, we do not know whether existing requirements will change or whether compliance with future requirements will require significant unanticipated expenditures that will adversely affect our cash flow and results from operations.

Some potential losses are not covered by insurance.

We carry insurance coverage on our properties of types and in amounts that we believe are in line with coverage customarily obtained by owners of similar properties. We believe all of our properties are adequately insured. The property insurance that we maintain for our properties has historically been on an “all risk” basis, which is in full force and effect until renewal in September 2007. There are other types of losses, such as from wars or catastrophic acts of nature, for which we cannot obtain insurance at all or at a reasonable cost. In the event of an uninsured loss or a loss in excess of our insurance limits, we could lose both the revenues generated from the affected property and the capital we have invested in the affected property. Depending on the specific circumstances of the affected property it is possible that we could be liable for any mortgage indebtedness or other obligations related to the property. Any such loss could adversely affect our business and financial condition and results of operations.

Also, we have to renew our policies in most cases on an annual basis and negotiate acceptable terms for coverage, exposing us to the volatility of the insurance markets, including the possibility of rate increases. Any material increase in insurance rates or decrease in available coverage in the future could adversely affect our results of operations and financial condition.

Potential liability for environmental contamination could result in substantial costs.

Under Federal, state and local environmental laws, ordinances and regulations, we may be required to investigate and clean up the effects of releases of hazardous or toxic substances or petroleum products at our properties, regardless of our knowledge or responsibility, simply because of our current or past ownership or operation of the real estate. In addition, the U.S. Environmental Protection Agency and the U.S. Occupational Safety and Health Administration are increasingly involved in indoor air quality standards, especially with respect to asbestos, mold and medical waste. The clean up of any environmental contamination, including asbestos and mold, can be costly. If unidentified environmental problems arise, we may have to make substantial payments which could adversely affect our cash flow, because:

These costs could be substantial and in extreme cases could exceed the value of the contaminated property. The presence of hazardous or toxic substances or petroleum products or the failure to properly remediate

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contamination may adversely affect our ability to borrow against, sell or rent an affected property. In addition, applicable environmental laws create liens on contaminated sites in favor of the government for damages and costs it incurs in connection with a contamination.

We have a storage tank third party liability, corrective action and cleanup policy in place to cover potential hazardous releases from underground storage tanks on our properties. This insurance is in place to mitigate any potential remediation costs from the effect of releases of hazardous or toxic substances from these storage tanks. Additional coverage is in place under a pollution legal liability real estate policy. This would, dependent on circumstance and type of pollutants discovered, provide further coverage above and beyond the storage tank policy.

Environmental laws also govern the presence, maintenance and removal of asbestos. Such laws require that owners or operators of buildings containing asbestos:

Such laws may impose fines and penalties on building owners or operators who fail to comply with these requirements and may allow third parties to seek recovery from owners or operators for personal injury associated with exposure to asbestos fibers.

It is our policy to retain independent environmental consultants to conduct Phase I environmental site assessments and asbestos surveys with respect to our acquisition of properties. These assessments generally include a visual inspection of the properties and the surrounding areas, an examination of current and historical uses of the properties and the surrounding areas and a review of relevant state, Federal and historical documents, but do not involve invasive techniques such as soil and ground water sampling. Where appropriate, on a property-by-property basis, our practice is to have these consultants conduct additional testing, including sampling for asbestos, for mold, for lead in drinking water, for soil contamination where underground storage tanks are or were located or where other past site usages create a potential environmental problem, and for contamination in groundwater. Even though these environmental assessments are conducted, there is still the risk that:

Recently enacted changes in securities laws are likely to increase our costs.

The Sarbanes-Oxley Act of 2002, as well as new rules subsequently implemented by the Securities and Exchange Commission, have required change in some of our corporate governance and accounting practices. In addition, the New York Stock Exchange has promulgated a number of regulations. We expect these laws, rules and regulations to increase our legal and financial compliance costs and to make some activities more difficult, time consuming and costly. We also expect these new rules and regulations to make it more difficult and expensive for us to obtain director and officer liability insurance, and we may be required to accept reduced coverage or incur significantly higher costs to obtain coverage. These new laws, rules and regulations could also make it more difficult for us to attract and retain qualified members of our board of trustees, particularly to serve on our audit committee, and qualified executive officers.

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We face risks associated with the use of debt to fund acquisitions and developments, including refinancing risk.

We rely on borrowings under our credit facilities to finance acquisitions and development activities and for working capital. If we were unable to borrow under our credit facilities, or to refinance existing indebtedness, our financial condition and results of operations would likely be adversely affected.

We are subject to the risks normally associated with debt financing, including the risk that our cash flow may be insufficient to meet required payments of principal and interest. We anticipate that only a small portion of the principal of our debt will be repaid prior to maturity. Therefore, we are likely to need to refinance at least a portion of our outstanding debt as it matures. There is a risk that we may not be able to refinance existing debt or that the terms of any refinancing will not be as favorable as the terms of the existing debt. If principal payments due at maturity cannot be refinanced, extended or repaid with proceeds from other sources, such as new equity capital, our cash flow will not be sufficient to repay all maturing debt in years when significant “balloon” payments come due.

Rising interest rates would increase our interest costs.

We may incur indebtedness that bears interest at variable rates. Accordingly, if interest rates increase, so will our interest costs, which could adversely affect our cash flow and our ability to service debt. As a protection against rising interest rates, we may enter into agreements such as interest rate swaps, caps, floors and other interest rate exchange contracts. These agreements, however, increase our risks including other parties to the agreements not performing or that the agreements may be unenforceable.

Covenants in our debt agreements could adversely affect our financial condition.

Our credit facilities contain customary restrictions, requirements and other limitations on our ability to incur indebtedness. We must maintain certain ratios, including total debt to assets, secured debt to total assets, debt service coverage and minimum ratios of unencumbered assets to unsecured debt. Our ability to borrow under our credit facilities is subject to compliance with our financial and other covenants.

Failure to comply with any of the covenants under our unsecured credit facilities or other debt instruments could result in a default under one or more of our debt instruments. This could cause our lenders to accelerate the timing of payments and would therefore have a material adverse effect on our business, operations, financial condition and liquidity.

Further issuances of equity securities may be dilutive to current shareholders.

The interests of our existing shareholders could be diluted if additional equity securities are issued to finance future developments and acquisitions instead of incurring additional debt. Our ability to execute our business strategy depends on our access to an appropriate blend of debt financing, including unsecured lines of credit and other forms of secured and unsecured debt, and equity financing.

Failure to qualify as a REIT would cause us to be taxed as a corporation, which would substantially reduce funds available for payment of dividends.

If we fail to qualify as a REIT for federal income tax purposes, we would be taxed as a corporation. We believe that we are organized and qualified as a REIT and intend to operate in a manner that will allow us to continue to qualify as a REIT.

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If we fail to qualify as a REIT we could face serious tax consequences that could substantially reduce the funds available for payment of dividends for each of the years involved because:

In addition, if we fail to qualify as a REIT, we would no longer be required to pay dividends. As a result of these factors, our failure to qualify as a REIT could impair our ability to expand our business and raise capital, and could adversely affect the value of our shares.

The market value of our securities can be adversely affected by many factors.

As with any public company, a number of factors may adversely influence the public market price of our common shares, many of which are beyond our control. These factors include:

[None]

The schedule on the following pages lists our real estate investment portfolio as of December 31, 2005, which consisted of 68 properties.

As of December 31, 2005, the percent leased is the percentage of net rentable area for which fully executed leases exist and may include signed leases for space not yet occupied by the tenant.

Cost information is included in Schedule III to our financial statements included in this Annual Report on Form 10-K.

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SCHEDULE OF PROPERTIES

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SCHEDULE OF PROPERTIES (continued)

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None.

No matters were submitted to a vote of security holders during the fourth quarter of 2005.

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PART II

Our shares trade on the New York Stock Exchange. Currently, there are approximately 47,300 shareholders.

The high and low sales price for our shares for 2005 and 2004, by quarter, and the amount of dividends we paid per share are as follows:

We have historically paid dividends on a quarterly basis. Dividends are normally paid based on our cash flow from operating activities.

During the period covered by this report, we did not sell any equity securities without registration under the Securities Act.

Neither we nor any affiliated purchaser (as that term is defined in Securities Exchange Act Rule 10b-18(a) (3)) made any repurchases of our shares during the fourth quarter of the fiscal years covered by this report.

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The discussion and analysis of our financial condition and results of operations are based upon our consolidated financial statements, which have been prepared in accordance with accounting principles generally accepted in the United States. The preparation of these financial statements requires us to make estimates and judgments that affect the reported amounts of assets, liabilities, revenues and expenses. On an on-going basis, we evaluate these estimates, including those related to estimated useful lives of real estate assets, estimated fair value of acquired leases, cost reimbursement income, bad debts, contingencies and litigation. We base the estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. There can be no assurance that actual results will not differ from those estimates.

Overview

Our revenues are derived primarily from the ownership and operation of income-producing real properties in the greater Washington/Baltimore region. As of December 31, 2005, we owned a diversified portfolio of 68 properties, consisting of 21 office buildings, 7 medical office buildings, 12 retail centers, 9 multifamily buildings and 19 industrial complexes totaling 10.3 million net rentable square feet. We also hold land for development. We have a fundamental strategy of regional focus, diversification by property type and conservative capital management.

When evaluating our financial condition and operating performance, management focuses on the following financial and non-financial indicators, discussed in further detail herein:

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During 2005 we continued our long-standing strategy of focusing in the greater Washington/Baltimore region, one of the most stable real estate markets in the country. The region posted positive job growth of approximately 3.0% in the twelve months ended November 2005 and an unemployment rate of 3.3% compared to the national average of 5.0%. The job growth occurred principally in the professional/business services, retail and construction sectors. This is a positive sign for continued economic growth in the region. Overall conditions in the region improved during the year, with continued strength in the retail sector and stabilizing rents in the office and industrial sectors, as the multifamily sector began to benefit from the overall market conditions.

Overall occupancies as well as our results in 2005 were primarily impacted by the $229.1 million in acquisitions we completed in 2004 and 2005, dispositions of $83.5 million and the performance of our core portfolio (consisting of properties owned for the entirety of 2005 and the same time period in 2004) which improved slightly compared to 2004.

The performance of our five operating segments generally reflected market conditions in our region:

During 2005 development continued on Phase I of Rosslyn Towers, our mixed-use residential and retail community in Virginia. We began development at South Washington Street and redevelopment at Foxchase Shopping center, both in Alexandria, Virginia and started the planning, late in the year, for the development of Dulles Station, acquired in December 2005.

Significant transactions during the two years ended December 31, 2005 are summarized below:

2005

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2004

CRITICAL ACCOUNTING POLICIES AND ESTIMATES

We believe the following critical accounting policies reflect the more significant judgments and estimates used in the preparation of our consolidated financial statements. Our significant accounting policies are described in Note 2 in the Notes to the Consolidated Financial Statements in Item 8 of this Form 10-K.

Revenue Recognition

Residential properties are leased under operating leases with terms of generally one year or less, and commercial properties are leased under operating leases with average terms of three to seven years. We recognize rental income and rental abatements from our residential and commercial leases when earned on a straight-line basis in accordance with SFAS No. 13 “Accounting for Leases.” Recognition of rental income commences when control of the facility has been given to the tenant. We record a provision for losses on accounts receivable equal to the estimated uncollectible amounts. This estimate is based on our historical experience and a review of the current status of the company’s receivables. Percentage rents, which represent additional rents based on gross tenant sales, are recognized when tenants’ sales exceed specified thresholds.

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In accordance with SFAS No. 66, “Accounting for Sales of Real Estate,” sales are recognized at closing only when sufficient down payments have been obtained, possession and other attributes of ownership have been transferred to the buyer and we have no significant continuing involvement.

We recognize cost reimbursement income from pass-through expenses on an accrual basis over the periods in which the expenses were incurred. Pass-through expenses are comprised of real estate taxes, operating expenses and common area maintenance costs which are reimbursed by tenants in accordance with specific allowable costs per tenant lease agreements.

Capital Expenditures

We capitalize those expenditures related to acquiring new assets, significantly increasing the value of an existing asset, or substantially extending the useful life of an existing asset. In addition, we capitalize tenant leasehold improvements when certain conditions are met, including when we supervise construction and will own the improvements. Expenditures necessary to maintain an existing property in ordinary operating condition are expensed as incurred.

Real Estate Assets

Real estate assets are depreciated on a straight-line basis over estimated useful lives ranging from 28 to 50 years. All capital improvement expenditures associated with replacements, improvements, or major repairs to real property are depreciated using the straight-line method over their estimated useful lives ranging from 3 to 30 years. All tenant improvements are amortized over the shorter of the useful life or the term of the lease.

We allocate the purchase price of acquired properties to the related physical assets and in-place leases based on their relative fair values, in accordance with SFAS No. 141, “Business Combinations.” The fair values of acquired buildings are determined on an “as-if-vacant” basis considering a variety of factors, including the physical condition and quality of the buildings, estimated rental and absorption rates, estimated future cash flows and valuation assumptions consistent with current market conditions. The “as-if-vacant” fair value is allocated to land, building and tenant improvements based on property tax assessments and other relevant information obtained in connection with the acquisition of the property.

The fair value of in-place leases consists of the following components—(1) the estimated cost to us to replace the leases, including foregone rents during the period of finding a new tenant, foregone recovery of tenant pass-throughs, tenant improvements, and other direct costs associated with obtaining a new tenant (referred to as “Tenant Origination Cost”); (2) estimated leasing commissions associated with obtaining a new tenant (referred to as “Leasing Commissions”); (3) the above/at/below market cash flow of the leases, determined by comparing the projected cash flows of the leases in place to projected cash flows of comparable market-rate leases (referred to as “Net Lease Intangible”); and (4) the value, if any, of customer relationships, determined based on our evaluation of the specific characteristics of each tenant’s lease and our overall relationship with the tenant (referred to as “Customer Relationship Value”).

The amounts used to calculate Tenant Origination Cost, Leasing Commissions, and Net Lease Intangible are discounted using an interest rate which reflects the risks associated with the leases acquired. Tenant Origination Costs are included in Real Estate Assets on our balance sheet and are amortized as depreciation expense on a straight-line basis over the remaining life of the underlying leases. Leasing Commissions are classified as Other Assets and are amortized as amortization expense on a straight-line basis over the remaining life of the underlying leases. The aggregate value of the cash flow for above market leases results in Net Lease Intangible Assets which are classified as Other Assets and are amortized on a straight-line basis as a decrease to Real Estate Rental Revenue over the remaining term of the underlying leases. The aggregate value of the cash flow for below market leases results in Net Lease Intangible Liabilities which are classified as Other Liabilities and are amortized on a straight-line basis as an increase to Real Estate Rental Revenue over the remaining term of the underlying leases. The aggregate value of the cash flow of leases at market results in no additional assets or liabilities.

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Discontinued Operations

We dispose of assets (sometimes using tax-deferred exchanges) that are inconsistent with our long-term strategic or return objectives and where market conditions for sale are favorable. The proceeds from the sales are reinvested into other properties, used to fund development operations or to support other corporate needs, or are distributed to our shareholders.

We classify properties as held for sale when they meet the necessary criteria specified by SFAS No. 144, “Accounting for the Impairment or Disposal of Long-Lived Assets”. These include: senior management commits to and actively embarks upon a plan to sell the assets, the sale is expected to be completed within one year under terms usual and customary for such sales and actions required to complete the plan indicate that it is unlikely that significant changes to the plan will be made or that the plan will be withdrawn. Depreciation on these properties is discontinued, but operating revenues, operating expenses and interest expense continue to be recognized until the date of sale.

Under SFAS No. 144, revenues and expenses of properties that are either sold or classified as held for sale are treated as discontinued operations for all periods presented in the Statements of Income.

Impairment Losses on Long-Lived Assets

We recognize impairment losses on long-lived assets used in operations when indicators of impairment are present and the net undiscounted cash flows estimated to be generated by those assets are less than the assets’ carrying amount. If such carrying amount is in excess of the estimated cash flows from the operation and disposal of the property, we would recognize an impairment loss equivalent to an amount required to adjust the carrying amount to the estimated fair market value. There were no property impairments recognized during the three-year period ended December 31, 2005.

Federal Income Taxes

We believe that we qualify as a REIT under Sections 856-860 of the Internal Revenue Code and intend to continue to qualify as such. To maintain our status as a REIT, we are required to distribute 90% of our ordinary taxable income to our shareholders. We have the option of (i) reinvesting the sale price of properties sold, allowing for a deferral of income taxes on the sale, (ii) paying out capital gains to the shareholders with no tax to the company or (iii) treating the capital gains as having been distributed to the shareholders, paying the tax on the gain deemed distributed and allocating the tax paid as a credit to the shareholders. We distributed 100% of our 2005 and 2004 ordinary taxable income, and the gains from the property disposed in 2004, to shareholders. $33.5 million of the gains from property disposed in 2005 was reinvested in replacement properties. Approximately $3.5 million of gains from disposed property in 2005 was distributed to shareholders. We distributed 100% of our 2003 ordinary taxable income to our shareholders. No provision for income taxes was necessary during the three year period ended December 31, 2005.

RESULTS OF OPERATIONS

The discussion that follows is based on our consolidated results of operations for the years ended December 31, 2005, 2004 and 2003. The ability to compare one period to another may be significantly affected by acquisitions completed and dispositions made during those years.

For purposes of evaluating comparative operating performance, we categorize our properties as “core”, “non-core” or Discontinued Operations. A “core” property is one that was owned for the entirety of the periods being evaluated and is included in continuing operations. A “non-core” property is one that was acquired during either of the periods being evaluated and is included in continuing operations. Results for properties sold or held for sale during any of the periods evaluated are classified as Discontinued Operations.

To provide more insight into our operating results, our discussion is divided into two main sections: (1) Consolidated Results of Operations where we provide an overview analysis of results on a consolidated basis and (2) Net Operating Income (“NOI”) where we provide a detailed analysis of core versus non-core property-level NOI results by segment. NOI is calculated as real estate rental revenue less real estate operating expenses.

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Consolidated Results of Operations

Real Estate Rental Revenue

Real Estate Rental Revenue is summarized as follows (all data in thousands except percentage amounts):

Real estate rental revenue is comprised of (1) minimum base rent, which includes rental revenues recognized on a straight-line basis, (2) revenue from the recovery of operating expenses from our tenants and (3) other revenue such as parking and termination fees and percentage rents.

Minimum base rent increased $15.1 million (9.7%) in 2005 as compared to 2004 and $15.9 million (11.4%) in 2004 as compared to 2003. The increase in minimum base rent in 2005 was due primarily to the increase in rent from properties acquired in 2004 and 2005 ($12.5 million), combined with a $2.6 million increase in minimum base rent from core properties due to lower vacancies and rental rate increases in the Retail, Industrial and Multifamily sectors. The increase in minimum base rent in 2004 was due primarily to the increase in rent from properties acquired in 2004 ($1.8 million) and 2003 ($13.1 million), combined with a $1.0 million increase in minimum base rent from core properties due to rental rate increases and lower vacancies in the Office and Retail sectors.

A summary of economic occupancy for properties classified as continuing operations by sector follows:

Consolidated Economic Occupancy

Our overall economic occupancy increased 100 basis points in 2005 as compared to 2004 and increased 80 basis points in 2004 as compared to 2003. Property acquisitions in the Industrial and Retail sectors and decreased vacancy in the Multifamily sector, partially offset by higher vacancies in the Office sector, accounted for the increase in overall economic occupancy in 2005. Acquisitions of highly occupied properties and increased Industrial leasing activity, partially offset by higher vacancies in the Multifamily and Retail sectors accounted for the increase in 2004. A detailed discussion of occupancy by sector can be found in the Net Operating Income section.

Recoveries from tenants increased $3.5 million (29.1%) in 2005 as compared to 2004 and $2.1 million (21.2%) in 2004 as compared to 2003. The increase in recoveries from tenants in 2005 was due primarily to increased recovery income from core properties ($1.7 million) due to higher operating expense, utilities, common area maintenance, real estate taxes and from properties acquired in 2005 and 2004 ($1.8 million). The increase in recoveries from tenants in 2004 was due primarily to recoveries from properties acquired in 2003 and 2004 ($0.8 million) and a $1.3 million increase in recoveries from core properties due to higher common area maintenance, real estate tax and operating expenses.

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Parking and other tenant charges decreased $0.2 million in 2005 as compared to 2004 and were flat in 2004 as compared to 2003. The decrease in parking and other charges in 2005 was driven by core properties due primarily to lower percentage rent.

Real estate operating expenses

Real estate operating expenses are summarized as follows (all data in thousands except percentage amounts):

Property operating expenses include utilities, repairs and maintenance, property administration and management, operating services, common area maintenance and other operating expenses.

Real estate operating expenses as a percentage of revenue were 30.6% for 2005, 29.9% for 2004 and 29.1% for 2003.

Properties acquired in 2004 and 2005 accounted for $2.1 million (43.5%) of the $4.9 million increase in 2005 property operating expenses. Core property operating expenses increased $2.9 million as a result of higher utility costs due largely to rate increases and energy taxes, increased administrative expenditures and higher repairs and maintenance costs. Real estate taxes increased $1.9 million due primarily to the properties acquired in 2004 and 2005, which accounted for $1.1 million (59.1%) of the increase. The remainder of the increase in real estate taxes was due primarily to higher value assessments among our core properties.

Property acquisitions in 2004 and 2003 accounted for $2.7 million (60%) of the $4.5 million increase in property operating expenses in 2004. Core property operating expenses increased $1.8 million as a result of higher utility costs due largely to rate increases and an increase in the Montgomery County, MD energy tax, increased security related expenditures and higher repairs and maintenance costs. Real estate taxes increased $2.2 million due primarily to property acquisitions, which accounted for $1.7 million (77%) of the increase. The remainder of the increase in real estate taxes was due primarily to higher value assessments among our core properties.

Other Operating Expenses

Other operating expenses are summarized as follows (all data in thousands except percentage amounts):

Depreciation and amortization

The $7.9 million increase in depreciation and amortization expense in 2005 relative to 2004 was due substantially to acquisitions of $145.1 million and $84.0 million in 2005 and 2004, respectively. The increase in depreciation and amortization expense attributable to 2005 and 2004 acquisitions combined was $5.1 million or 64.7% of the total $7.9 million increase.

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The $5.8 million increase in depreciation and amortization expense in 2004 as compared to 2003 was due primarily to acquisition activity in both years. The increase in depreciation and amortization expense attributable to 2004 and 2003 acquisitions combined was $4.9 million (84%) of the total $5.8 million increase, due to the $84.0 million and $174.4 million acquisitions in 2004 and 2003, respectively.

Interest expense

The $3.2 million increase in interest expense in 2005 as compared to 2004 was primarily due to (1) the issuance in April 2005 of $50.0 million of 5.05% senior unsecured notes due May 1, 2012 and $50.0 million of 5.35% senior unsecured notes due May 1, 2015 and in October 2005, the issuance of an additional $100.0 million of notes of the series of 5.35% senior unsecured notes due May 1, 2015, (2) the increase in short-term borrowing on our lines of credit to fund acquisitions and (3) the mortgage assumption of $24.3 million (fair value of $25.0 million) in March 2005 for the acquisition of Frederick Crossing Shopping Center. The increase to interest expense as a result of this activity ($7.5 million) was partially offset by lower interest expense of $3.9 million due to repayment of $55.0 million of 7.78% unsecured notes in November 2004, and higher capitalized interest on development projects of $0.4 million.

The $4.5 million increase in interest expense in 2004 as compared to 2003 was primarily due to (1) the issuance of $100.0 million in 5.25% unsecured notes in December 2003 to refinance short-term borrowings used to fund a portion of the 1776 G Street and Prosperity Medical Center acquisitions, (2) the issuance of $60.0 million in 5.125% unsecured notes in March 2003, (3) the assumption of $49.8 million in mortgages in October 2003 for the acquisition of Prosperity Medical Center and (4) the assumption of $29.6 million in mortgages in 2004 for the acquisitions of Shady Grove Medical Village II and Dulles Business Park. The increase to interest expense as a result of this activity ($7.7 million) was partially offset by lower interest expense of $2.2 million due to the payoff of $50.0 million of 7.125% unsecured notes in August 2003 and $0.5 million due to the repayment of $55.0 million of 7.78% unsecured notes in November 2004, and higher capitalized interest on development projects of $0.4 million.

A summary of interest expense for the years ended December 31, 2005, 2004, and 2003 appears below (in millions):