The National Capital Planning Commission rejected a controversial plan yesterday by the U.S. Postal Service and its private New York development partners to add 1 1/2 stories atop the historic former main post office next to Union Station, leaving the future of the project uncertain.

The 7-to-2 vote came after months of intense lobbying by postal officials and the developers in what emerged as a testy battle among several federal agencies. The plan's rejection is considered a major victory for local preservationist groups who argued that the proposed 33-foot addition would destroy the character of the massive, 73-year-old Beaux Arts structure, as well as the surrounding historic area.

The commission, the central planning agency for the federal government in the Washington area, reviews a wide array of government and private development plans, including anything that might have an impact on federal land and buildings. It is composed of representatives from several federal agencies, Congress and the District, as well as private persons appointed by the president.

"We have sent them a message," said commission Chairman Glen T. Urquhart, whose influential opposition to the proposal led to its defeat. "I believe it sets the stage for the post office to return with a modified {building development} proposal without the 1 1/2-story addition."

Urquhart, other board members who voted against the measure and the preservationist groups said they still favor some form of refurbishing of the District's old central mail-handling facility, which has sat mostly idle for more than a year at Massachusetts Avenue and North Capitol Street.

But the NCPC's denial casts a cloud over what will happen with the historic structure, which was last renovated during the Eisenhower administration and has served as a modest bookend to Union Station since 1914. Postal officials from the agency's real estate division and representatives from the New York development firms handling the project declined to comment after the board's contentious two-hour hearing.

"We have no comment until we assess what happened," said Meg Harris, a Postal Service spokeswoman.

As part of a $100 million renovation, the Postal Service and its private development partners -- a joint venture of the New York real estate firms Julien J. Studley Inc. and Arthur G. Cohen Properties Inc. -- proposed last year transforming the building into a huge complex of offices, shops and a postal museum. They used the museum component as a bargaining chip, arguing consistently that any museum would be contingent solely on the approval of the addition, which would have added more than 350,000 square feet of space to the structure, an amount larger than most new downtown office buildings.

Without the additional rental space the proposed 1 1/2 stories would have created, the project's backers had questioned the economic feasibility of refurbishing the building.

Groups opposed to the addition have contended that any increased building height would distort the vista of the Columbus Circle area, as well as the postal building, which was designed by noted Union Station architect Daniel Burnham. Critics also claimed that the increased building space and the additional workers housed in the complex would create further traffic snarls and parking nightmares in the already congested area.

"I had not expected it. We are quite pleased," said Michael Quinn, executive director of the D.C. Preservation League, one of the groups that fought the proposed addition. "It shows that the Postal Service had its priorities out of line."

Dorn McGrath, chairman of the Committee of 100 on the Federal City, a local planning watchdog group, called the addition "an inferior architectural solution to a questionable building development program."

He said approval of the project would have "set an extremely dangerous precedent" for other federal buildings in Washington.

The staff of the commission, whose recommendation for denial of the project was accepted by the board, said, "It is essential for the federal government to exemplify the highest standards of preservation, rather than just mere compliance, in the treatment and use of its own buildings."

The two board members who voted for the Postal Service proposal are employed by two federal agencies that were lobbied heavily during the last month by postal officials and representatives from the development companies eager to win approval for their plan.

"I think it's an outstanding opportunity for the city," said Richard M. Hadsell, a General Service Administration representative on the commission who voted for the plan. GSA has also been negotiating with the Postal Service for a possible federal government lease for part of the building's space if the project is eventually carried out.

"We do have an interest in the case," Hadsell acknowledged.

John G. Parsons, the Interior Department's representative on the commission, also voted for the addition. He declined to comment on the case.

Yesterday's meeting got off to a shaky start when Whayne Quin, an attorney for the Postal Service and the developers, asked that the board delay its decision for a month or more.

His bid backfired when Urquhart declared that the panel, after dealing with nearly six months of various delays in the case, would decide the issue at yesterday's meeting.

Quin, after a brief huddle with the postal officials and developers, then withdrew their development application.

But the commission went ahead with the case anyway, hearing opposition arguments from the staff and preservationist groups.

"We have not been given due process," Quin told the board, a charge several commission members angrily rejected.