JOINDER AGREEMENT

October 12, 2005

Reference is made to the Revolving Credit Agreement, dated as of November 30,
2004 (as from time to time amended and in effect, the “Loan Agreement”), among
First Potomac Realty Investment Limited Partnership (“FPLP”) and each other
Borrower (collectively, the “Borrower”) which from time to time is a party to
the Loan Agreement, KeyBank National Association (“KeyBank”) and the other
lending institutions referred to in the Loan Agreement as Lenders (collectively,
the “Lenders”), and KeyBank, as managing administrative agent for itself and
each other Lender (the “Agent”). Capitalized terms used herein and not otherwise
defined shall have the meanings assigned to such in the Loan Agreement.

In consideration of and as an inducement to the inclusion by the Lenders of each
of the Real Estate Asset(s) identified on Exhibit A hereto as an Eligible
Unencumbered Property pursuant to the Loan Agreement, each of FP Campostella
Road, LLC, a Delaware limited liability company, and FP Diamond Hill, LLC, a
Delaware limited liability company (each, an “Additional Borrower” and
collectively, the “Additional Borrowers”), each of which is a Wholly-owned
Subsidiary of FPLP, hereby acknowledges and agrees to the terms and conditions
of the Loan Agreement, the Revolving Credit Notes and the other Loan Documents
to which any Borrower is a party, joins in the agreements of the Borrower under
the Loan Agreement, the Revolving Credit Notes and the other Loan Documents to
which any Borrower is a party and agrees that all Obligations of the Borrower
under the Loan Agreement, the Revolving Credit Notes and the other Loan
Documents to which any Borrower is a party shall be the obligations, jointly and
severally, of the Additional Borrowers and the Borrower with the same force and
effect as if each Additional Borrower was originally a Borrower under the Loan
Agreement and an original signatory to the Loan Agreement, the Revolving Credit
Notes and the other Loan Documents to which any Borrower is a party.

Each Additional Borrower further agrees that its liability hereunder is direct
and primary and may be enforced by the Lenders and the Agent before or after
proceeding against any other Borrower.

At least three (3) Business Days prior to this Joinder Agreement becoming
effective and each of the Real Estate Asset(s) identified in Exhibit A hereto
becoming an Eligible Unencumbered Property pursuant to the Loan Agreement, each
Additional Borrower shall have delivered to the Agent (with copies to the Agent
for each Lender) the documents and other items required to be delivered pursuant
to Section 8.13(c), 12.2, 12.3, 12.4, 12.8 and 12.13 of the Loan Agreement, in
each case in form and substance satisfactory to the Agent, along with such other
documents, certificates and instruments reasonably required by the Agent,
including, if necessary, updates to the schedules to the Loan Agreement
satisfactory to the Agent. Without in any way limiting the other rights of the
Agent under the Loan Agreement, each Additional Borrower agrees that the Agent
shall have the right to visit and inspect such Eligible Unencumbered Property at
the Borrower’s sole cost and expense.

Each of the undersigned represents and warrants to the Agent and the Lenders
that it has the complete right, power and authority to execute and deliver this
Joinder Agreement and to perform all of the obligations hereunder and the
Obligations under the Loan Agreement, the Revolving Credit Notes and the other
Loan Documents to which any Borrower is a party. This Joinder Agreement shall be
binding upon the undersigned and its successors and assigns and shall inure to
the benefit of the Lenders, the Agent and their respective successors and
assigns.

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Executed as a sealed instrument as of the 12th day of October, 2005.

FP CAMPOSTELLA ROAD, LLC,
a Delaware limited liability company

By: _/s/ Barry Bass     
Barry Bass, Executive Vice President and Chief
Financial Officer

FP DIAMOND HILL, LLC,

a Delaware limited liability company

By: _/s/ Barry Bass     
Barry Bass, Executive Vice President and Chief
Financial Officer

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Acknowledged and Agreed:

 
 
FIRST POTOMAC REALTY INVESTMENT LIMITED PARTNERSHIP, for itself and as agent
for each other Borrower
 
By: First Potomac Realty Trust, its
sole general partner
 
By:      /s/ Barry Bass      
 
Executive Vice President and
Chief Financial Officer

Acknowledged:
KEYBANK NATIONAL ASSOCIATION,
Individually and as Managing Administrative Agent
under the Loan Agreement

By: _/s/ John Scott     
Name: John Scott
Title: Vice President

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Exhibit A to Joinder Agreement

  1.   1960 Diamond Hill Road, Chesapeake, Virginia 23324

  2.   1910 Campostella Road, Chesapeake, Virginia 23324

  3.   1920 Campostella Road, Chesapeake, Virginia 23324

  4.   2115 Portlock Road, Chesapeake, Virginia 23324

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