The Washington Convention Center Authority has reached an agreement to buy a half-acre parcel of land at the corner of Massachusetts Avenue and Ninth Street NW that is crucial to its plans for developing a 1,220-room hotel near the new convention center, a city official said yesterday.

The deal includes a historic 90-year-old building and a small lot owned by the United Association of Journeymen and Apprentices of the Plumbing and Pipe Fitting Industry, said the official, who is close to the negotiations but spoke on the condition of anonymity because the transaction has not yet closed.

Earlier this month, the authority made the plumbers union an offer for the land and promised a $900,000 deposit on the property. The full amount has not been disclosed. The offer was prompted after the plumbers union got an unsolicited $30 million offer from a Philadelphia-based real estate company.

The land is part of a block the authority and the District are trying to acquire to build what would be the city's largest hotel next to the new convention center. Meeting planners say having a major hotel next to the convention center would be a draw for delegates.

Much of the rest of the block is owned by D.C. developer Kingdon Gould III. The city is close to negotiating a deal with Gould to swap his land at Ninth and Massachusetts in exchange for a small piece of land two blocks away at the site of the old convention center, at Ninth and New York Avenue NW. The D.C. Council must approve the land deal with Gould.

Early next month, the authority is expected to release a consultant's report on the proposed hotel and how to pay for it.

City officials said they are talking with Marriott International Inc. of Bethesda and Robert L. Johnson, former owner of Black Entertainment Television Inc., about a deal to pay for the $400 million hotel complex with private money instead of using public funds.

The city has selected Hines Interests LP of Houston and Archstone-Smith Trust of Englewood, Colo., to turn the roughly 10 acres at the old convention center site into a mixed-use development. The plans call for construction of about 700 condominium and apartment units; about 275,000 square feet of retail; 300,000 square feet of offices; and a library, theater or other cultural venue. The project is expected to be completed in 2011.