PRESS RELEASE DETAIL


May 27, 2011

CoStar Group, Inc. Announces Exercise of Underwriters' Over-Allotment Option to Purchase an Additional 562,500 Shares of Common Stock

WASHINGTON, May 27, 2011 -- CoStar Group, Inc. (Nasdaq:CSGP) announced today that the underwriters have exercised their option to purchase an additional 562,500 shares of CoStar's common stock at $60.00 per share, in addition to the 3,750,000 shares of CoStar common stock, which were offered on May 25, 2011, pursuant to an underwriting agreement. J.P. Morgan Securities LLC is the sole book-running manager, with Needham & Company, LLC, Stephens Inc., William Blair & Company, L.L.C. and JMP Securities LLC as co-managers for the offering. The offering is expected to close on June 1, 2011, subject to customary closing conditions.

WASHINGTON, May 27, 2011 -- CoStar Group, Inc. (Nasdaq:CSGP) announced today that the underwriters have exercised their option to purchase an additional 562,500 shares of CoStar's common stock at $60.00 per share, in addition to the 3,750,000 shares of CoStar common stock, which were offered on May 25, 2011, pursuant to an underwriting agreement. J.P. Morgan Securities LLC is the sole book-running manager, with Needham & Company, LLC, Stephens Inc., William Blair & Company, L.L.C. and JMP Securities LLC as co-managers for the offering. The offering is expected to close on June 1, 2011, subject to customary closing conditions.

CoStar expects to use the net proceeds of the offering to fund a portion of the cash consideration payable in connection with its acquisition of LoopNet, Inc. and, to the extent that any proceeds remain thereafter, or the acquisition is not completed, for general corporate purposes. The offering is not conditioned on the closing of the LoopNet acquisition.

The shares are being offered pursuant to an effective shelf registration statement that has been filed with the Securities and Exchange Commission (SEC). A preliminary prospectus supplement related to the offering has been filed with the SEC and is available on the SEC's website at http://www.sec.gov or from the Company at http://www.costar.com/investors.aspx (under the SEC Filings tab). Copies of the prospectus supplement and accompanying prospectus relating to the offering, when available, may be obtained from: J.P. Morgan Securities LLC, c/o Broadridge Financial Solutions, 1155 Long Island Avenue, Edgewood, NY 11717, Telephone 866-803-9204.

This news release shall not constitute an offer to sell or the solicitation of an offer to buy, nor shall there be any sale of these securities in any state or jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state or jurisdiction.

About CoStar Group, Inc.
CoStar Group (Nasdaq:CSGP) is commercial real estate's leading provider of information and analytic services. Founded in 1987, CoStar conducts expansive, ongoing research to produce and maintain the largest and most comprehensive database of commercial real estate information. Our suite of online services enables clients to analyze, interpret and gain unmatched insight on commercial property values, market conditions and current availabilities. Headquartered in Washington, DC, CoStar maintains offices throughout the U.S. and in Europe with a staff of approximately 1,500 worldwide, including the industry's largest professional research organization. For more information, visit www.costar.com.


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