The Washington Convention Center Authority voted unanimously yesterday to place a $900,000 deposit on property at the corner of Massachusetts Avenue and Ninth Street NW, one of the crucial pieces it would need to build the District's largest hotel.

The decision comes a week after the United Association of Journeymen and Apprentices of the Plumbing and Pipe Fitting Industry, which owns the land and the 90-year-old building that sits on it, reportedly received a $30 million offer from Lubert-Adler Management Inc., a Philadelphia-based real estate company, according to Jim Abdo, who heads the authority's development committee.

Executives at Lubert-Adler and the plumbers union did not return phone calls seeking comment.

The city wants to acquire much of the block at Ninth Street and Massachusetts Avenue NW to build a 1,220-room hotel, next to the new convention center at Mount Vernon Square. In addition to the plumbers' half-acre, the city would need to buy a 11/2-acre property and a few smaller parcels, Abdo said.

"If we didn't get the plumbers site, it would offer a wrinkle" in building the hotel, he said. "It's a significantly crucial piece. This is a good-faith deposit to say, 'We're here and we believe in this site for a hotel and expansion space for the convention center."

He said the offer to the plumbers union allows for a 60-day study period so members of the D.C. Council and the authority can review a soon-to-be-released consultant's report on the hotel and how to pay for it.

Abdo would not say exactly how much the authority, which oversees the operations of the convention center, would pay for the union's land but said it "intends to be competitive with an offer that they allegedly have in hand."

He said he has talked to the union's real estate broker but not to leaders at the union. "We're hearing that from their representatives," he said of the reported $30 million offer from Lubert-Adler, which he said wanted to build condominiums on the site. "We haven't seen anything. We expect they'll make a decision and we hope they recognize our offer as a very solid, credible one."

Abdo said the hotel's design would take into account the historic building and work around it.

City Council Chairman Linda W. Cropp (D) had fought to have the hotel put on land at the old convention center site, two blocks south. But the mayor and the authority say they favored the union's property because it is next to the convention center and conventioneers prefer to have a large, headquarters hotel next to where they meet.

The plumbers union owns half an acre at Ninth Street and Massachusetts Avenue. Developer Kingdon Gould III owns the rest of that block, about 11/2 acres.

The city is negotiating with Gould to swap his land for a portion of city-owned land at the old convention center. The convention center authority is expected to reach a deal with Gould on the swap in the coming weeks and expects to present the deal to the D.C. Council, which must approve it, this fall.

Marriott International Inc. of Bethesda and Robert L. Johnson, former owner of Black Entertainment Television, are negotiating a deal to possibly pay for the $400 million hotel complex.