Developer JBG Cos. is in "serious negotiations" with D.C.-based Corporate Executive Board Co. over a long-term lease for a 24-story office tower that JBG plans to build in Rosslyn, according to sources close to the negotiations.

If it goes through, the deal for the 620,000-square-foot building would be the biggest lease signed this year in the D.C. region, brokers and developers said. It would provide a critical major tenant for JBG's planned Waterview project, which will also include 183 condominiums and a 156-room hotel.

JBG and its partners hope to complete the deal in the next month, according to the sources, who spoke on condition of anonymity because the negotiations are confidential. The building is expected to be completed in three years.

An official at Corporate Executive Board would not comment yesterday except to say that the company has looked at the Waterview and other sites for its 1,250 employees. Corporate Executive Board provides business research and analysis to executives in such areas as human resources, auditing and information technology. The publicly traded company has been growing rapidly and is now spread out in four buildings in the District, according to Timothy R. Yost, the company's chief financial officer.

A deal with Corporate Executive Board would be "a home run" for the Waterview project, said Kathryn Schmitt, research manager at brokerage firm Grubb & Ellis. "The Corporate Executive Board is the kind of client who wants their own building and they don't have to be downtown."

Chevy Chase-based JBG plans a million-square-foot complex on the site overlooking the Potomac River. A one-bedroom condo is expected to sell for $500,000; and a 1,100-square-foot two-bedroom condo for roughly $1 million. Two restaurants and shops will also be built.

The three buildings that sit on the site, at the corner of North 19th and North Lynn streets, will be torn down by the end of the year, and construction on the project will start next year. The project is expected to be completed in mid-2007 and will cost about $300 million to build. JBG's partners in the deal are Trizec Properties Inc., a Chicago-based real estate investment trust, and CIM Group, a pension fund adviser in Los Angeles.

The biggest lease so far this year in the region is the U.S. Environmental Protection Agency's deal in May to take more than 400,000 square feet in a residential, office and retail complex under construction at the Potomac Yard site, just south of Crystal City.