In what would be the city's largest office lease yet this year, the law firm Piper Rudnick LLP tentatively agreed to be the major tenant in a large office building planned for the East End.

The 3/4-acre site at Ninth and E streets NW is now a parking lot next to the International Spy Museum and one of the few undeveloped parcels in the city zoned for high-rise office development, said city officials, real estate brokers and developers.

Having such a big tenant would let developer Boston Properties Inc. proceed, said senior vice president Jonathan L. Kaylor. Boston Properties plans to start construction on the 10-story, 300,000-square-foot building next summer and to open it in 2007. The developer expects to spend at least $120 million on construction and land.

Piper Rudnick, still negotiating terms, would lease about 200,000 square feet over 15 years. Kaylor said he expects to lease the rest to other law firms or lobbyists.

Piper Rudnick said it has outgrown the 160,000 square feet it leases at 1200 19th St. NW downtown and needs more space for its 180 lawyers and 300 support staffers. "We like the location on that side of town, where everything is going on," said Jay A. Epstien, a lawyer and spokesman for the firm.

Large developers have long desired the site. Part of the site is owned by Jan Evans Houser, a longtime Washingtonian and an heir to the defunct Heurich Brewing Co. in Foggy Bottom, which made beer from 1873 until it went out of business in 1955.

Houser said her grandfather bought a corner of the lot where a saloon once sat that served his beer. She and Irwin Edlavitch, who she said owns the rest of the land, did not want to sell, so they went into partnership with Boston Properties.

"You just don't sell things," Evans said. "The land value goes up. If we have a dip now, you just hold it and it will go up again."

Boston Properties is a real estate investment trust based in Boston with revenue of $1.3 billion last year. It is the fourth-largest office developer in the Washington region.

The project joins a rising amount of development downtown. Some developers and brokers wonder how much demand there is to match it.

Since 2000, 8 million square feet of office space has been built downtown -- most of it in the East End, where blocks of abandoned houses and used-car lots from 16th Street to Union Station meant there was lots of land available. The city's office vacancy rate doubled from 6 percent in early 2000 to 12.1 percent in the first quarter of this year.

"There's over 4 million square feet of office space under construction," said Kathryn Schmitt, research manager at brokers Grubb & Ellis Co., "but as long as pre-leases are getting signed like this, it's not overbuilt."

The second-largest lease this year was taken by another law firm, White & Case LLP, which moves this fall into 100,000 square feet at One Metro Center, a large building that opened last year at 701 13th St. NW, according to Grubb & Ellis.

Law firms are the city's second-largest tenants after the government, and like the government they help keep the city's economy relatively healthy even when the national economy drags.