<PAGE>

                                                                   EXHIBIT 10.10

                       MEMBERSHIP INTEREST SALE AGREEMENT

                                 BY AND BETWEEN

                          CLARK/CARR INVESTMENTS, LLC,
                      A MARYLAND LIMITED LIABILITY COMPANY,

                                    AS SELLER

                                       AND

                              COLUMBIA EQUITY, LP,
                         A VIRGINIA LIMITED PARTNERSHIP,

                                  AS PURCHASER

<PAGE>

                                TABLE OF CONTENTS

<TABLE>
<S>                                                                                              <C>
ARTICLE I  THE SALE........................................................................       3

        1.1      Sale of Membership Interest...............................................       3
        1.2      Purchase Price............................................................       3

ARTICLE II  REPRESENTATIONS AND COVENANTS..................................................       4

        2.1      Representations by Purchaser..............................................       4
        2.2      Representations by Seller.................................................       5
        2.3      Covenants of Purchaser....................................................       6
        2.4      Covenants of Seller.......................................................       6

ARTICLE III  CONDITIONS PRECEDENT TO THE CLOSING...........................................       7

        3.1      Conditions to Purchaser's Obligations.....................................       7
        3.2      Conditions to Seller's Obligations........................................       7

ARTICLE IV  CLOSING AND CLOSING DOCUMENTS..................................................       8

        4.1      Closing...................................................................       8
        4.2      Seller's Deliveries.......................................................       8
        4.3      Purchaser's Deliveries....................................................       9
        4.4      Fees and Expenses; Closing Costs..........................................       9
        4.5      Adjustments...............................................................       9

ARTICLE V  MISCELLANEOUS...................................................................       9

        5.1      Notices...................................................................       9
        5.2      Entire Agreement; Modifications and Waivers; Cumulative Remedies..........      10
        5.3      Exhibits..................................................................      10
        5.4      Successors and Assigns....................................................      10
        5.5      Article Headings..........................................................      11
        5.6      Governing Law.............................................................      11
        5.7      Counterparts..............................................................      11
        5.8      Survival..................................................................      11
        5.9      Severability..............................................................      11
        5.10     Attorneys' Fees...........................................................      11
</TABLE>

EXHIBITS

      A     Assignment and Assumption Agreement

<PAGE>

                       MEMBERSHIP INTEREST SALE AGREEMENT

      THIS MEMBERSHIP INTEREST SALE AGREEMENT (this "Agreement") is made as of
this 31st day of January, 2005 by and between Clark/Carr Investments, LLC, a
Maryland limited liability company ("Seller"); and Columbia Equity, LP, a
Virginia limited partnership ("Purchaser").

                                    RECITALS

      A. Meadows IV, LLC, a Virginia limited liability company (the "LLC") is
the owner of certain land located in Chantilly, Virginia (the "Land") and the
office building and related improvements located thereon (the "Improvements"),
which Land and Improvements (collectively, the "Property") are more commonly
known as the Meadows IV Building.

      B. Meadows IV Investors, LLC, a Virginia limited liability company
("Meadows IV Investors") is the direct and indirect owner of One Hundred and
00/100 percent (100.00%) of the membership interests in the LLC.

      C. Seller is the record and beneficial owner of Forty-Three and 60/100
percent (43.60%) of the membership interests in Meadows IV Investors (the
"Membership Interest").

      D. Seller desires to sell the Membership Interest to Purchaser, on the
terms and conditions hereinafter set forth.

      E. Purchaser desires to purchase the Membership Interest from Seller, on
the terms and conditions hereinafter set forth.

                                    AGREEMENT

      NOW, THEREFORE, for and in consideration of the mutual covenants herein
contained, the parties hereto agree as follows:

                                    ARTICLE I
                                    THE SALE

      1.1 Sale of Membership Interest. Seller agrees to sell, transfer, assign
and convey the Membership Interest to Purchaser, and Purchaser agrees to
purchase and accept transfer of the Membership Interest pursuant to the terms
and conditions set forth in this Agreement. The Membership Interest shall be
transferred to Purchaser free and clear of all liens, encumbrances, security
interests, prior assignments or conveyances, conditions, restrictions, voting
agreements, claims, and any other matters affecting title thereto (other than
the LLC's Operating Agreement (the "LLC Operating Agreement")).

      1.2 Purchase Price. The purchase price (the "Purchase Price") for which
Seller agrees to sell and assign the Membership Interest to Purchaser, and which
Purchaser agrees to pay to Seller, subject to the terms of this Agreement, shall
be equal to an amount providing the Seller with a twenty percent (20%) IRR (as
defined in Meadows IV Investors' Operating Agreement (the "Meadows IV Investors
Operating Agreement")), based on a minimum of a six month investment holding
period for Seller's Capital Contribution (as defined in the Meadows IV

                                      -3-
<PAGE>

Investors Operating Agreement) (that is, based on initial Capital Contribution
of $2,212,500, assuming no distributions by the Company, the minimum Purchase
Price would equal $2,443,176).

                                   ARTICLE II
                          REPRESENTATIONS AND COVENANTS

      2.1 Representations by Purchaser. Purchaser hereby represents and warrants
to Seller that the following statements are true, correct, and complete in every
material respect as of the date of this Agreement and will be true, correct, and
complete as of the Closing Date:

            (a) Organization and Power. Purchaser is duly organized and validly
existing as a limited partnership under the laws of the Commonwealth of
Virginia, and has full right, power, and authority to enter into this Agreement
and to perform all of its obligations under this Agreement; and, the execution
and delivery of this Agreement and the performance by Purchaser of its
obligations under this Agreement have been duly authorized by all requisite
action of Purchaser and require no further action or approval of Purchaser's
partners or of any other individuals or entities in order to constitute this
Agreement as a binding and enforceable obligation of Purchaser.

            (b) Noncontravention. Neither the entry into nor the performance of,
or compliance with, this Agreement by Purchaser has resulted, or will result, in
any violation of, or default under, or result in the acceleration of, any
obligation under the partnership agreement of Purchaser, or any mortgage,
indenture, lien agreement, note, contract, permit, judgment, decree, order,
restrictive covenant, statute, rule, or regulation applicable to Purchaser.

            (c) Litigation. There is no action, suit, or proceeding, pending or
known to be threatened, against or affecting Purchaser in any court or before
any arbitrator or before any federal, state, municipal, or other governmental
department, commission, board, bureau, agency or instrumentality which (i) in
any manner raises any question affecting the validity or enforceability of this
Agreement, (ii) would reasonably be expected to materially and adversely affect
the business, financial position, or results of operations of Purchaser, (iii)
would reasonably be expected to materially and adversely affect the ability of
Purchaser to perform its obligations hereunder, or under any document to be
delivered pursuant hereto.

            (d) Consents. Each consent, approval, authorization, order, license,
certificate, permit, registration, designation, or filing by or with any
governmental agency or body necessary for the execution, delivery, and
performance of this Agreement or the transactions contemplated hereby by
Purchaser has been obtained.

            (e) Bankruptcy with respect to Purchaser. No Act of Bankruptcy has
occurred with respect to Purchaser. As used herein, "Act of Bankruptcy" shall
mean if a party hereto shall (A) apply for or consent to the appointment of, or
the taking of possession by, a receiver, custodian, trustee or liquidator of
itself or of all or a substantial part of its property, (B) admit in writing its
inability to pay its debts as they become due, (C) make a general assignment for
the benefit of its creditors, (D) file a voluntary petition or commence a
voluntary case or proceeding under the Federal Bankruptcy Code (as now or
hereafter in effect), (E) be adjudicated bankrupt or insolvent, (F) file a
petition seeking to take advantage of any other law relating to bankruptcy,

                                       -4-
<PAGE>

insolvency, reorganization, winding-up or composition or adjustment of debts,
(G) fail to controvert in a timely and appropriate manner, or acquiesce in
writing to, any petition filed against it in an involuntary case or proceeding
under the Federal Bankruptcy Code (as now or hereafter in effect), or (H) take
any action for the purpose of effecting any of the foregoing.

            (f) Brokerage Commission. Purchaser has not engaged the services of,
nor has it or will it or Seller become liable to, any real estate agent, broker,
finder or any other person or entity for any brokerage or finder's fee,
commission or other amount with respect to the transactions described herein on
account of any action by Purchaser. Purchaser hereby agrees to indemnify and
hold Seller and its employees, directors, members, partners, affiliates and
agents harmless against any actual claims, liabilities, damages or expenses
arising out of a breach of the foregoing. This indemnification shall survive
Closing or any termination of this Agreement.

      2.2 Representations by Seller. Seller hereby represents and warrants unto
Purchaser that each and every one of the following statements is true, correct,
and complete in every material respect as of the date of this Agreement and will
be true, correct, and complete as of the Closing Date:

            (a) Organization and Power. Seller is duly organized, validly
existing, and in good standing as a limited liability company under the laws of
the State of Maryland. Seller has full right, power, and authority to enter into
this Agreement and to perform all of its obligations under this Agreement; and
the execution and delivery of this Agreement and the performance by Seller of
its obligations hereunder have been duly authorized by all requisite action of
Seller and require no further action or approval of Seller's members or managers
or of any other individuals or entities in order to constitute this Agreement as
a binding and enforceable obligation of Seller.

            (b) Noncontravention. To Seller's actual knowledge, neither the
entry into nor the performance of, or compliance with, this Agreement by Seller
has resulted, or will result, in any material violation of, or material default
under, or result in the acceleration of, any material obligation under any
material limited liability company agreement, operating agreement, regulation,
mortgage, indenture, lien agreement, note, contract, permit, judgment, decree,
order, restrictive covenant, statute, rule, or regulation applicable to Seller
or, to the actual knowledge of Seller, to the Membership Interest.

            (c) Litigation. There is no action, suit, claim, or proceeding
pending or, to the actual knowledge of Seller, threatened against Seller, or, to
the actual knowledge of Seller, the Membership Interest in any court, or before
any arbitrator, or before any federal, state, municipal or other governmental
department, commission, board, bureau, agency or instrumentality which (A) would
reasonably be expected to materially and adversely affect the ability of Seller
to perform its obligations hereunder, or under any document to be delivered
pursuant hereto, (B) would reasonably be expected to create a lien on the
Membership Interest, any part thereof, or any interest therein, or (C) would
reasonably be expected to materially and adversely affect the Membership
Interest, any part thereof, or any interest therein.

            (d) Good Title. (A) Seller has good title to the Membership Interest
on the date hereof and will have good title to the Membership Interest on the
Closing Date (other than the LLC Operating Agreement), (B) the Membership
Interest on the date hereof is and on the Closing Date will be free and clear of
all liens, encumbrances, pledges, voting agreements and

                                      -5-
<PAGE>

security interests whatsoever (other than the LLC Operating Agreement), and (C)
Seller has not granted any other person or entity an option to purchase or a
right of first refusal upon the Membership Interest nor are there any agreements
or understandings between Seller and any other person or entity with respect to
the disposition of the Membership Interest (other than the LLC Operating
Agreement).

            (e) Bankruptcy with respect to Seller. No Act of Bankruptcy has
occurred with respect to Seller.

            (f) Brokerage Commission. Seller has not engaged the services of,
nor has it or will it or Purchaser become liable to, any real estate agent,
broker, finder or any other person or entity for any brokerage or finder's fee,
commission or other amount with respect to the transactions described herein on
account of any action by Seller. Seller hereby agrees to indemnify and hold
Purchaser and its employees, directors, members, partners, affiliates and agents
harmless against any actual claims, liabilities, damages or expenses arising out
of a breach of the foregoing. This indemnification shall survive Closing or any
termination of this Agreement.

            (g) Default. To Seller's actual knowledge, Seller is not in default
under Meadows IV Investors Operating Agreement or the LLC Operating Agreement.

      2.3 Covenants of Purchaser. Purchaser agrees as follows:

            (a) Further Acts. In addition to the acts, instruments and
agreements recited herein and contemplated to be performed, executed and
delivered by Purchaser and Seller, Purchaser shall perform, execute, and deliver
or cause to be performed, executed, and delivered at the Closing or after the
Closing, any and all further acts, instruments, and agreements and provide such
further assurances as Seller may reasonably require to consummate the
transactions contemplated hereunder.

      2.4 Covenants of Seller. Seller agrees as follows:

            (a) Actions Regarding Membership Interest. Except as otherwise
permitted hereby, from the date hereof until the Closing Date, Seller shall use
reasonable commercial efforts not to take any action or fail to take any action
within Seller's control the result of which would (1) have a material adverse
effect on the Membership Interest, Seller's ability to sell, transfer, assign
and convey the Membership Interest to Purchaser or Purchaser's ability to
continue the ownership thereof after the Closing Date or (2) cause any of the
representations and warranties contained in Section 2.2 to be untrue in any
material respects as of the Closing Date.

            (b) Confidentiality. Seller acknowledges that the matters relating
to the REIT, the initial public offering of REIT securities (the "IPO"), this
Agreement, and the other documents, terms, conditions and information related
thereto (collectively, the "Information") are confidential in nature. Therefore,
Seller covenants and agrees to keep the Information confidential and will not
(except as required by applicable law, regulation or legal process, and only
after compliance with the provisions of this Section 2.4) prior to the IPO,
without Purchaser's prior written consent, disclose any Information in any
manner whatsoever; provided, however, that the Information may be revealed only
to Seller's employees, legal counsel and

                                      -6-
<PAGE>

financial advisors, each of whom shall be informed of the confidential nature of
the Information. In the event that Seller or its key employees, legal counsel or
financial advisors (collectively, the "Information Group") are requested
pursuant to, or required by, applicable law, regulation or legal process to
disclose any of the Information, the applicable member of the Information Group
will notify Purchaser promptly so that it may seek a protective order or other
appropriate remedy or, in its sole discretion, waive compliance with the terms
of this Section 2.4. Seller acknowledges that Purchaser's sole remedy for breach
of this Section shall be to seek an order granting injunctive relief in favor of
the REIT and/or Purchaser without proof of actual damages.

            (c) Further Acts. In addition to the acts, instruments and
agreements recited herein and contemplated to be performed, executed and
delivered by Purchaser and Seller, Seller shall perform, execute, and deliver or
cause to be performed, executed, and delivered, at no additional cost to Seller,
at the Closing or after the Closing, any and all further acts, instruments, and
agreements and provide such further assurances as Purchaser may reasonably
require to consummate the transactions contemplated hereunder.

                                   ARTICLE III
                       CONDITIONS PRECEDENT TO THE CLOSING

      3.1 Conditions to Purchaser's Obligations. In addition to any other
conditions set forth in this Agreement, Purchaser's obligation to consummate the
Closing is subject to the timely satisfaction of each and every one of the
conditions and requirements set forth in this Section 3.1, all of which shall be
conditions precedent to Purchaser's obligations under this Agreement.

            (a) Seller's Obligations. Seller shall have performed all
obligations of Seller hereunder which are to be performed prior to Closing, and
shall have delivered or caused to be delivered to Purchaser, all of the
documents and other information required of Seller pursuant to Section 4.2.

            (b) Seller's Representations and Warranties. Seller's
representations and warranties set forth in Section 2.2 shall be true and
correct in all material respects as if made again on the Closing Date, and
Seller shall have executed and delivered to Purchaser at Closing a certificate
to the foregoing effect.

            (c) No Injunction. On the Closing Date, there shall be no effective
injunction, writ, preliminary restraining order or other order issued by a court
of competent jurisdiction restraining or prohibiting the consummation of the
transactions contemplated hereby.

            (d) Completion of IPO. The IPO shall have been completed.

            (e) Closing. The Closing shall have occurred on or prior to June 30,
2005.

      3.2 Conditions to Seller's Obligations. In addition to any other
conditions set forth in this Agreement, Seller's obligations to consummate the
Closing is subject to the timely satisfaction of each and every one of the
conditions and requirements set forth in this Section 3.2, all of which shall be
conditions precedent to Seller's obligations under this Agreement.

                                      -7-
<PAGE>

            (a) Purchaser's Obligations. Purchaser shall have performed all
obligations of Purchaser hereunder which are to be performed prior to Closing,
and shall have delivered or caused to be delivered to Seller, all of the
documents and other information required of Purchaser pursuant to Section 4.3.

            (b) Purchaser's Representations and Warranties. Purchaser's
representations and warranties set forth in Section 2.1 shall be true and
correct in all material respects as if made again on the Closing Date, and
Purchaser shall have executed and delivered to Seller at Closing a certificate
to the foregoing effect.

            (c) No Injunction. On the Closing Date, there shall be no effective
injunction, writ, preliminary restraining order or other order issued by a court
of competent jurisdiction restraining or prohibiting the consummation of the
transactions contemplated hereby.

            (d) Completion of IPO. The IPO shall have been completed.

            (e) Loans. Loans. The mezzanine loan secured by Meadows IV
Investors' interest in the LLC (the "Mezzanine Loan") shall be paid off on the
Closing Date.

            (f) Closing. The Closing shall have occurred on or prior to June 30,
2005.

                                   ARTICLE IV
                          CLOSING AND CLOSING DOCUMENTS

      4.1 Closing. The consummation and closing (the "Closing") of the
transactions contemplated under this Agreement shall take place at the offices
of Hunton & Williams LLP, Washington, D.C., or such other place as is mutually
agreeable to the parties, on the date of the closing of the IPO (the "Closing
Date"), or as otherwise set by agreement of the parties; provided, however, that
this Agreement shall terminate if Closing does not occur prior to June 30, 2005.

      4.2 Seller's Deliveries. At the Closing, Seller shall deliver the
following to Purchaser in addition to all other items required to be delivered
to Purchaser by Seller:

            (a) Assignment of Membership Interest. Seller shall have executed
and delivered to Purchaser an Assignment and Assumption Agreement, in
substantially the form of Exhibit A attached hereto (the "Assignment and
Assumption Agreement").

            (b) Authority Documents. Evidence satisfactory to Purchaser that the
person or persons executing the closing documents on behalf of Seller has full
right, power, and authority to do so.

            (c) FIRPTA Certificate. An affidavit from Seller certifying pursuant
to Section 1445 of the Internal Revenue Code that Seller is not a foreign
corporation, foreign partnership, foreign trust, foreign estate or foreign
person (as those terms are defined in the Internal Revenue Code and the Income
Tax Regulations promulgated thereunder), in form and substance satisfactory to
Purchaser.

                                      -8-
<PAGE>

            (d) Certificate of Representations and Warranties. The certificate
required by Section 3.1(b).

            (e) Other Documents. Any other document or instrument reasonably
requested by Purchaser or required hereby.

      4.3 Purchaser's Deliveries. At the Closing, Purchaser shall deliver the
following to Seller:

            (a) Purchase Price. The Purchaser shall deliver funds in the amount
of the Purchase Price to Seller.

            (b) Assumption of Membership Interest. Purchaser shall have executed
and delivered to Seller an Assignment and Assumption Agreement.

            (c) Authority Documents. Evidence satisfactory to Seller that the
person or persons executing the closing documents on behalf of Purchaser have
full right, power, and authority to do so.

            (d) Certificate of Representations and Warranties. The certificate
required by Section 3.2(b).

            (e) Other Documents. Any other document or instrument reasonably
requested by Seller or required hereby.

      4.4 Fees and Expenses; Closing Costs. Purchaser shall pay all fees,
expenses and closing costs relating to the transactions contemplated by this
Agreement; provided however, that Seller shall pay its own attorneys' and
consultants' fees and expenses.

      4.5 Adjustments.

            (a) Purchaser shall pay funds in the amount of the Purchase Price to
Seller without any setoffs or adjustments. The LLC shall be managed in
accordance with the LLC Operating Agreement in the ordinary course of business,
up to and including the Closing Date.

            (b) Purchaser shall pay off the Mezzanine Loan on the Closing Date.
However, Purchaser shall be solely responsible (and shall not receive any credit
against the Purchase Price) for any prepayment penalties or lender fees paid in
connection with the paying off of the Mezzanine Loan.

                                   ARTICLE V
                                  MISCELLANEOUS

      5.1 Notices. Any notice provided for by this Agreement and any other
notice, demand, or communication required hereunder shall be in writing and
either delivered in person (including by confirmed facsimile transmission) or
sent by registered or certified mail or overnight courier, return receipt
requested, in a sealed envelope, postage prepaid, and addressed to the party for
which such notice, demand or communication is intended at such party's address
as set forth in this Section. All notices to Purchaser shall be addressed as
follows:

                                      -9-
<PAGE>

      PURCHASER:

            Columbia Equity, LP
            c/o Carr Capital Corporation
            1750 H Street, NW, Suite 500
            Washington, DC  20005
            Telephone: (202) 303-3060
            Facsimile: (202) 303-3078
            Attn: Oliver T. Carr, III

Seller's address for all purposes under this Agreement shall be the following:

      SELLER:

            Clark/Carr Investments, LLC
            c/o CEI Realty, Inc.
            7500 Old Georgetown Road, 15th Floor
            Bethesda, MD  20814
            Attn: Rebecca L. Owen, Esq.

Any address or name specified above may be changed by a notice given by the
addressee to the other party. Any notice, demand or other communication shall be
deemed given and effective as of the date of delivery in person or receipt set
forth on the return receipt. The inability to deliver because of changed address
of which no notice was given, or rejection or other refusal to accept any
notice, demand or other communication, shall be deemed to be receipt of the
notice, demand or other communication as of the date of such attempt to deliver
or rejection or refusal to accept.

      5.2 Entire Agreement; Modifications and Waivers; Cumulative Remedies. This
Agreement supersedes any existing letter of intent between the parties,
constitutes the entire agreement among the parties hereto and may not be
modified or amended except by instrument in writing signed by the parties
hereto, and no provisions or conditions may be waived other than by a writing
signed by the party waiving such provisions or conditions. No delay or omission
in the exercise of any right or remedy accruing to Seller or Purchaser upon any
breach under this Agreement shall impair such right or remedy or be construed as
a waiver of any such breach theretofore or thereafter occurring. The waiver by
Seller or Purchaser of any breach of any term, covenant, or condition herein
stated shall not be deemed to be a waiver of any other breach, or of a
subsequent breach of the same or any other term, covenant, or condition herein
contained. All rights, powers, options, or remedies afforded to Seller or
Purchaser either hereunder or by law shall be cumulative and not alternative,
and the exercise of one right, power, option, or remedy shall not bar other
rights, powers, options, or remedies allowed herein or by law, unless expressly
provided to the contrary herein.

      5.3 Exhibits. All exhibits referred to in this Agreement and attached
hereto are hereby incorporated in this Agreement by reference.

      5.4 Successors and Assigns. Except as set forth in this Article, this
Agreement may not be assigned by Purchaser or Seller without the prior approval
of the other party hereto. This

                                      -10-
<PAGE>

Agreement shall be binding upon, and inure to the benefit of, Seller, Purchaser,
and their respective legal representatives, successors, and permitted assigns.

      5.5 Article Headings. Article headings and article and section numbers are
inserted herein only as a matter of convenience and in no way define, limit, or
prescribe the scope or intent of this Agreement or any part hereof and shall not
be considered in interpreting or construing this Agreement.

      5.6 Governing Law. This Agreement shall be construed and interpreted in
accordance with the laws of the Commonwealth of Virginia, without regard to
conflicts of laws principles.

      5.7 Counterparts. This Agreement may be executed in any number of
counterparts and by any party hereto on a separate counterpart, each of which
when so executed and delivered shall be deemed an original and all of which
taken together shall constitute but one and the same instrument.

      5.8 Survival. All representations and warranties contained in this
Agreement, and all covenants and agreements contained in the Agreement which
contemplate performance after the Closing Date shall survive the Closing.

      5.9 Severability. In case any one or more of the provisions contained in
this Agreement shall for any reason be held to be invalid, illegal, or
unenforceable in any respect, such invalidity, illegality, or unenforceability
shall not affect any other provision hereof, and this Agreement shall be
construed as if such invalid, illegal, or unenforceable provision had never been
contained herein.

      5.10 Attorneys' Fees. Should a party employ an attorney or attorneys to
enforce any of the provisions hereof or to protect its interest in any manner
arising under this Agreement, or to recover damages for breach of this
Agreement, any non-prevailing party in any action pursued in a court of
competent jurisdiction (the finality of which is not legally contested) shall
pay to the prevailing party all reasonable costs, damages, and expenses,
including reasonable attorneys' fees, expended or incurred in connection
therewith.

      5.11 Section 8.4 Rights. Seller hereby waives its rights under Section 8.4
of the Meadows IV Investors Operating Agreement during the term of this
Agreement.

                                      -11-
<PAGE>

      IN WITNESS WHEREOF, this Agreement has been entered into effective as of
the date first above written.

                                   SELLER:

                                   Clark/Carr Investments, LLC, a Maryland
                                   limited liability company

                                   By: CEI Realty, Inc., a Maryland corporation,
                                       its manager

                                       By: /s/ Lawrence C. Nussdorf
                                          --------------------------------------
                                       Name:
                                       Title:

                                   PURCHASER:

                                   Columbia Equity, LP, a Virginia limited
                                   partnership

                                   By: Columbia Equity Trust, Inc., a Maryland
                                       corporation, its general partner

                                       By: /s/ Oliver T. Carr, III
                                           -------------------------------------
                                       Name: Oliver T. Carr, III

                                       Title: Chairman and CEO

                                      -12-
<PAGE>

                                    EXHIBIT A

                       ASSIGNMENT AND ASSUMPTION AGREEMENT

Clark/Carr Investments LLC, a Maryland Limited liability company ("Assignor"),
for good and valuable consideration paid to the Assignor by Columbia Equity, LP,
a Virginia limited partnership ("Assignee"), pursuant to the Membership Interest
Sale Agreement dated as of ____________, 2005, by and between
_______________________ (the "Agreement") and for other good and valuable
consideration, the receipt and adequacy of which are hereby acknowledged, does
hereby sell, assign, transfer, convey and deliver to the Assignee, its
successors and assigns, good and indefeasible title to the Membership Interest,
free and clear of all liens, encumbrances, security interests, prior
assignments, voting agreements, conditions, restrictions, pledges, claims, and
other matters affecting title thereto, subject to the LLC Operating Agreement.
Assignee does hereby accept the foregoing Assignment and assumes and agrees to
be responsible for all liabilities and obligations under the LLC Operating
Agreement from and after the date hereof relating to the Membership Interest.

Assignee agrees to indemnify, hold harmless and defend Assignor from and against
all claims, damages, debts, liabilities and obligations arising from business
done, transactions entered into or other events occurring after the date hereof
with respect to the Membership Interest, the Meadows IV Investors Operating
Agreement or the LLC Operating Agreement.

Capitalized terms used but not defined herein shall have the respective meanings
ascribed to them in the Agreement.

IN WITNESS WHEREOF, the parties hereto have caused this Assignment and
Assumption Agreement to be signed by a duly authorized officer of each, this __
day of _____, 2005.

                                   ASSIGNOR:

                                   _________________________, a

                                   _______________________

                                   By: _________________________________________
                                   Name: _______________________________________
                                   Title: ______________________________________

                                   ASSIGNEE:

                                   _________________________, a

                                   _______________________

                                   By: _________________________________________
                                   Name: _______________________________________
                                   Title: ______________________________________