Exhibit
23.1
Consent
of Independent Registered Public Accounting Firm
We
consent to the incorporation by reference in Registration Statements 333-82599,
333-92165, 333-45770, 333-69548, and 333-135709 of our report dated February
19,
2007 (June 22, 2007 as to the change in operating segments described in Note
14)
with respect to the consolidated financial statements of CoStar Group, Inc.
included in this Current Report on Form 8-K of CoStar Group, Inc. dated June
22,
2007.
/s/
Ernst
& Young LLP
McLean,
Virginia
June
22,
2007