Exhibit 10.1

THE FRANCIS G. NEWLANDS BUILDING

ADDENDUM NO. 4 TO OFFICE LEASE

THIS ADDENDUM NO. 4 TO OFFICE LEASE (this “Addendum No. 4”) is made and entered
into this 2nd day of April, 2008, by and between NEWLANDS BUILDING VENTURE, LLC,
a Maryland limited liability company, hereinafter called “Lessor”, and COSTAR
REALTY INFORMATION, INC., a Delaware corporation, hereinafter called “Lessee”.

WITNESSETH:

WHEREAS, by Office Lease dated August 12, 1999, as amended by Addendum No. 1
dated May 15, 2000 and Addendum No. 2 dated July 1, 2000, Lessor leased to
Lessee approximately 60,041 square feet of rentable area (the “Original Demised
Premises”) on the ninth (9th), tenth (10th) and eleventh (11th) floors of the
office building situated at Two Bethesda Metro Center, Bethesda, Maryland (the
“Building”), together with certain storage space therein;

WHEREAS, pursuant to Addendum No. 3 dated May 12, 2004 (the “Addendum No. 3”),
and said Office Lease as so amended by Addenda Nos. 1-3, collectively, the
“Lease”), Lessor leased to Lessee additional space on the fourth (4th) floor of
the Building comprising approximately 13,522 square feet of rentable area (the
“Additional Space”); and

WHEREAS, Lessee desires to terminate the Lease with respect to the Additional
Space, and Lessor is willing to agree to such early termination with respect to
the Additional Space, on the terms and conditions set forth herein.

NOW, THEREFORE, in consideration of the mutual covenants and agreements
hereinafter set forth, the parties hereto do mutually agree that the Lease shall
be and is hereby amended to provide as follows, all capitalized terms being as
defined in the Lease unless otherwise noted herein:

1.  
TERMINATION OF ADDITIONAL SPACE

The term of the Lease with respect to the Additional Space is terminated
effective as of 11:59 p.m. on April 27, 2008 (the “Partial Termination Date”),
with the same force and effect as if such date were the expiration date under
the Lease with respect to the Additional Space.  From and after the Partial
Termination Date, the remaining premises leased by Lessee pursuant to the Lease
shall be the Original Demised Premises, and all references in the Lease to
“Demised Premises” shall mean and refer to the Original Demised Premises.  On or
before 11:59 p.m. on the Partial Termination Date, Lessee shall surrender the
Additional Space to Lessor free and clear of all occupancies and tenancies and
in good order, repair and condition, broom clean and free of all personal
property, furnishings and debris, and otherwise in accordance with the
provisions of the Lease.  
 

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If Lessee fails to surrender the Additional Space as required herein on or
before the Partial Termination Date, Lessee shall be a holdover tenant with
respect to the Additional Space and the terms and conditions of Section 33 of
the Lease, including without limitation Lessee’s liability to pay to Lessor the
amounts specified therein, shall apply to any holdover occupancy of the
Additional Space.

Lessee has advised Lessor that it has made arrangements with the new tenant of
the Additional Space, General Electric Capital Corporation (“GECC”), whereby
Lessee would leave the existing furniture in the Additional Space and GECC would
acquire same.  Accordingly, so long as GECC does not object, Lessee may
surrender the Additional Space with the existing furniture remaining.

2.  
RENT

Subject to the terms and conditions of this Addendum No. 4, Lessee shall have no
obligation to pay “Additional Monthly Rent” pursuant to Section 3 of Addendum
No. 3, accruing with respect to any period from and after the Partial
Termination Date.

3.  
RENTAL ESCALATION FOR INCREASES IN EXPENSES

Subject to the terms and conditions of this Addendum No. 4, Lessee shall have no
obligation to pay additional rent with respect to Lessee’s Additional Space
Proportionate Share of increases in Operating Expenses and increases in
Operating Costs accruing with respect to any period from and after the Partial
Termination Date.

4.  
BROKER AND AGENT

Lessor and Lessee each represents and warrants one to another that it has not
employed any broker, agent or finder in carrying on the negotiations, nor had
any dealings with any broker, agent or finder relating to this Addendum No.
4.  Lessor shall indemnify and hold Lessee harmless, and Lessee shall indemnify
and hold Lessor harmless, from and against any claims or claims for brokerage or
other commission arising from or out of any breach of the foregoing
representation and warranty by the respective indemnitors.

5.  
PARKING

Effective as of the Partial Termination Date, Lessee and its employees shall
have no further right to use any parking contracts entered into by Lessee in
connection with the leasing of the Additional Space pursuant to the Addendum No.
3, and same shall be terminated automatically.

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6.  
CONTINGENCY.

This Addendum No. 4 is contingent upon receipt by Lessor of a lease addendum
from GECC with respect to the leasing of the Additional Space, in form and
substance satisfactory to Lessor in its sole discretion (the “GE Addendum”).  If
Lessor does not receive the GE Addendum on or before April 3, 2008, this
Addendum No. 4 shall be null and void ab initio.  On or before April 4, 2008,
Lessor shall notify Lessee or Lessee’s broker (by email or other written
correspondence) whether or not Lessor received the GE Addendum.  At 9:00 a.m. on
April 28, 2008, Lessor and Lessee shall inspect the Additional Space with GECC.

7.  
FACSIMILE/COUNTERPART EXECUTION

 
Execution and delivery of this Addendum No. 4 by facsimile or pdf shall be
binding on the parties with the same effect as delivery of an original
signature.  This Addendum No. 4 may be executed in counterparts, which together
shall constitute a single instrument.
 

8.  
LEASE PROVISIONS APPLICABLE

Except to the extent expressly modified by this Addendum No. 4, all of the terms
and conditions of the Lease, as amended or supplemented hereby, shall remain in
full force and effect.

[SIGNATURE PAGES FOLLOW]

 
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IN WITNESS WHEREOF, Lessor and Lessee have caused this Addendum No. 4 to be
signed in their names by their duly authorized representatives and delivered as
their act and deed, intending to be legally bound by its terms and provisions.

LESSOR:
 
NEWLANDS BUILDING VENTURE, LLC
a Maryland limited liability company
 
By:           The Chevy Chase Land Company
 of Montgomery County, Maryland,
Attest:                                                                                
Managing Member
        
      /s/ illegible                       By:  /s/ Michele H. Cornwell
Secretary                                                                    
Name:  Michele H. Cornwell
(SEAL)                                                                        Title: 
Senior Vice President
 
LESSEE:
 
Attest:                                                                   COSTAR
REALTY INFORMATION, INC.
a Delaware corporation
 
       /s/ Jonathan Coleman

By:  /s/ Brian Radecki
Secretary                                                                     Name: 
Brian Radecki
(SEAL)                                                                Title: CFO
 

 
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STATE OF MARYLAND
COUNTY OF MONTGOMERY, ss:
 
I, Maoca V..O. Piehuta, a Notary Public in and for the jurisdiction aforesaid,
do hereby certify that Michele H. Cornwell Sr. VP, who is personally well known
to me as the person who executed the foregoing and annexed Addendum No. 4, dated
the 2nd day of April 2008, on behalf of Lessor, to acknowledge the same,
personally appeared before me in said jurisdiction and acknowledged said
Addendum No. 4 to be the act and deed of The Chevy Chase Land Company of
Montgomery County, Maryland, as Managing Member of and for and on behalf of the
Lessor, and delivered the same as such.
 
GIVEN under my hand and seal this 2nd day of April, 2008.
 

      /s/ Maoca V.O. Piehuta    
                            Notary Public
My commission expires:  8/13/2011
 

STATE OF MARYLAND
COUNTY OF MONTGOMERY,  ss:
 
I, Lisa Springer, a Notary Public in and for the jurisdiction aforesaid, do
hereby certify that Brian Radecki, who is personally well known to me to be the
person who executed the foregoing and annexed Addendum No. 4, dated the 2nd day
of April, 2008 on behalf of Lessee, to acknowledge the same, personally appeared
before me in said jurisdiction and acknowledged said Addendum No. 4 to be the
act and deed of CoStar Realty Information, Inc., and delivered the same as such.
 
GIVEN under my hand and seal this 2nd day of April, 2008.
 
 

      /s/ Lisa Springer    
                            Notary Public
My commission expires:  January 7, 2009
 

 
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