Corporate Executive Board Co., a District-based company, has signed a lease on a new headquarters in Rosslyn, a deal that would be the largest commercial real estate transaction in the region this year, according to industry sources close to the deal.

Sources, who asked not to be identified because the negotiations are confidential, said some details are still being worked out.

The publicly traded company, provides business research and analysis to executives in areas such as human resources, auditing and information technology. It has been growing rapidly, and its roughly 1,200 employees are spread out in four offices in the District.

Arlington County economic development officials said they are talking to Corporate Executive Board about incentives to get it to move to Rosslyn but would not elaborate. D.C. economic development officials said they also have offered Corporate Executive Board incentives to stay.

Under the 20-year Rosslyn lease, Corporate Executive Board would take 625,000 square feet of space at Waterview -- a large office, retail and housing complex being built by JBG Cos. of Chevy Chase. JBG's project, which will cost an estimated $300 million, will also have 8,000 square feet of retail space, 183 condominiums and a 156-room hotel overlooking the Potomac River.

Demolition of existing 1960s-era buildings is expected to start this fall, and construction of the 24-story office tower would start in March 2005, according to sources. The office building would be complete in February 2008. JBG's partners in the deal are Trizec Properties Inc., a Chicago real estate investment trust, and CIM Group, a pension fund adviser in Los Angeles.

An official at Corporate Executive Board would not comment on the deal. "We are always evaluating our long-term space needs and options," said Timothy R. Yost, the company's chief financial officer. "But as a matter of policy, we won't comment on any particular space."

Cushman & Wakefield Inc., the brokerage that represents Corporate Executive Board, would not comment on the transaction, nor would JBG. Benjamin Jacobs of JBG said yesterday that he disagreed with the details of the deal but would not elaborate.

"It's a monumental transaction because of its size," said Brendan R. Owen, senior vice president at CB Richard Ellis, which is not involved in the deal. High rents in the District have made Rosslyn attractive, Owen said. Rents for a high-end office building in downtown D.C. typically average in the low $60s a square foot, compared with the low $40s in Rosslyn.