A controversial plan by the U.S. Postal Service and its private development partners to add 1 1/2 stories atop the historic former District post office building has stumbled again, forcing the project's backers to withdraw from a crucial hearing.

In the latest setback, the National Capital Planning Commission's staff recommended that the proposal be rejected because of the addition's height and the traffic congestion that the massive rehabilitation project would create around Union Station.

The NCPC board was scheduled to consider the project this Thursday, but the Postal Service withdrew its application last week after learning of the staff's recommendation. Support from the NCPC staff is considered essential for developers seeking the NCPC board's approval.

The NCPC has authority to approve the plan because the building, located at North Capitol Street and Massachusetts Avenue NE, is on federal land. Reginald Griffith, executive director of NCPC, said the board supports renovation of the building in some form.

Last week's move by the Postal Service is the third time in three months that it has pulled out of an NCPC hearing because of fears that the plan would be killed. Nevertheless, postal officials said they will continue to seek approval for the project. "It was a surprise, and it is something we'll have to deal with," said Dennis Wamsley, the Postal Service's real estate manager. "But we're committed to this project."

Earlier this year, the Postal Service, in partnership with a group of private New York developers, proposed adding 2 1/2 stories onto the beaux arts-style building as part of a $100 million renovation project. In all, the project's backers planned to add more than 250,000 square feet of office and retail space to the city's once-bustling central mail-handling facility.

The building must be enlarged, the Postal Service and its development partners said, in order to make the project work economically. In their bid to get approval, they have promised to install a national postal museum inside the renovated building -- an offer contingent solely upon obtaining the additional square footage in the proposed addition.

But numerous civic groups and historic preservation supporters oppose the plan, arguing that the rooftop addition would destroy the 74-year-old building's character. Preservation leaders said the addition would overpower the surrounding area and would add to traffic and parking congestion in the area.

Because of the criticism, the Postal Service revised its plan in June, lopping off 11 feet from the proposed structure and leaving a new design that would add 1 1/2 stories to the building's current roof line.

Nonetheless, the D.C. Historic Preservation Review Board, in an influential yet nonbinding opinion, unanimously rejected the plans by the Postal Service and its private partners. The partners include the New York development firms of Julien J. Studley Inc. and Arthur G. Cohen Properties Inc. It was the second time that the preservation review board has rejected the plan.

Despite the review board's decision, the Postal Service decided to carry its case to the NCPC without modifications. But NCPC staff members were just as unforgiving.

"We think {the addition} should be reduced so that it does not project beyond the ridge line of the existing building," said Griffith of the NCPC. He said any new space needed for the project could be added to the building's core.

Despite the NCPC's staff recommendation, Griffith said, the NCPC board still favors renovating the building. "The concept is an excellent one," he said.

Stephen Porter, an attorney representing the New York developers, would not comment specifically on Griffith's objections. "It's nice that Reginald Griffith wants to try his case in the newspaper," he said.

Porter called the project "sensitively designed." "We continue to try to meet the legitimate objections of every interest group involved here in order to produce a project the city and country can be proud of, yet economically works," he said.