The D.C. Historic Preservation Review Board, in a nonbinding yet politically influential ruling, this week unanimously rejected for the second time a proposal by the U.S. Postal Service to construct a rooftop addition to the historic former main city post office next to Union Station.

The board's ruling, after an intense 3 1/2-hour meeting, was hailed as a victory for preservationists, who had argued that the proposed addition for the building at North Capital Street and Massachusetts Avenue NE would destroy the character of the stately old structure.

"No matter how sensitively crafted, any rooftop addition to the present historic building would . . . compromise its integrity as a significant architectural achievement," said Dorn McGrath Jr., chairman of the Committee of 100 on the Federal City, which opposed the addition.

The Postal Service, working in partnership with a group of private New York developers, earlier this year proposed adding 2 1/2 stories onto the building as part of a $100 million renovation project that would transform the city's once-bustling mail handling facility into an enormous complex with more than 1 million square feet of offices, shops and a postal museum.

However, the proposed addition was unanimously opposed by the historic review board three months ago. One board member at the time said the increased height would be "overpowering" to the surrounding area, particularly Union Station.

Last week, the developers revised their plan by lopping 11 feet off the proposed structure, leaving an addition that would rise 1 1/2 stories above the roofline of the 74-year-old beaux arts style building.

At Wednesday's review board hearing, postal officials and the project's developers said their revision was a "compromise." They further argued that if the addition was rejected, plans to include a national postal museum in the project would not be carried out -- a bargaining chip that has been employed throughout the testy debate.

"Quite frankly, {the museum} is completely conditional upon the approval of the addition," said Dennis Wamsley, real estate manager at the U.S. Postal Service. He said the new office space created by the addition would fund the museum, which he said the Postal Service otherwise could not afford.

Wamsley and other backers of the project, including New York development firms Julien J. Studley Inc. and Arthur G. Cohen Properties Inc., said the addition would complement the postal building not detract from it. Shalom Baranes, the project's architect, said the addition, which would be set back 200 feet from the Massachusetts Avenue side of the building, would be "very subtle."

But the review board's members, replaying a scene from last March, once again rejected the addition outright. Board member Harrison Ethridge said the old city post office "is one of the most important buildings in this city. It is not like any other post office in America. Any sort of rooftop addition would be inappropriate . . . and architecturally wrong."

Ethridge, embracing the views of McGrath and the Committee of 100, also said that approving the rooftop addition could set a dangerous precedent.

"It could be used as justification to add stories to buildings around the Capitol," Ethridge said. "I don't think we want to open up that possibility."

Colden Florance, a review board member who characterized himself as "project oriented," nevertheless joined the board in voting down the addition. "It's very clear to me that it doesn't work," he said.

The postal building's developers were hoping to obtain approval from the historic preservation review board to use as leverage before the National Capital Planning Commission, which has final authority in the case. The NCPC is scheduled to hear the proposal on July 2.

However, Charles Reiss, a partner with Housing Futures Inc., another of the project's New York developers, said after the review board's ruling that he had "no idea" if the group will proceed to the NCPC next month with their 31-foot addition proposal.

"We felt very confident about this proposal. I don't see how it's at all possible to meet the {review board's} resolution," said Reiss. He criticized the board's ruling "as incorrect for the city's own economy."

In siding with the preservationist groups, the review board rejected recommendations to approve the project from the city's preservation staff, the Commission of Fine Arts and Architect of the Capitol George M. White.

In addition, Fred L. Greene, the District's planning director, reversed his opposition to the project two months ago. Greene, who said in April that the original addition would "destroy a good urban design," said this week that the developer's new proposal is "responsive to the city's declaration on the preservation of historic structures."

The postal building, with its impressive white marble exterior and grand lobby area, was designed in 1913 by Daniel Burnham, a noted architect of the "city beautiful" movement, who also designed Union Station. The massive structure, built in two stages, has been deemed eligible as a landmark on the National Register of Historic Sites.

Michael Quinn, executive director of the D.C. Preservation League, said in opposing the addition "the city post office should be treated with better respect."