The old stone Post Office building has always been an orphan in the Federal Triangle.

The granite Romanesque structure, with its detailed cornices, fancy filigree and statuesque clock tower, made its debut on Pennsylvania Avenue in 1899. But the postal service that gave the building its name rejected the place only a dozen years after the doors opened, when it decided that the vaulted interior courtyard was not suited to sorting mail.

The building later was relegated to catching spillover government offices, rather than claiming an entire agency as a new namesake. Before the days of the Metrobus and the subway, D.C. Transit bus riders used to transfer there.

But now the old building, is about to take center stage in the resurgence taking place along the avenue. Come July, the former Post Office will attempt a renaissance akin to those enjoyed by Boston's Faneuil Hall and Manhattan's Southstreet Seaport.

What will emerge when the $20 million, 3-year renovation is completed is a mixed-use structure serving government and private business. The Post Office is the first major undertaking made possible by the 1976 Cooperative Use Act, which calls for the involvement of private enterprise in opening government buildings for public use.

The renovation, designed by Washington architect Arthur Cotton Moore, has the first three floors dramatically cut away, with a massive skylight topping the 10-story courtyard.

The lower floors will be occupied by about 50 restaurants and boutiques overlooking a central courtyard dominated by a stage. Upstairs, the offices will be occupied primarily by the National Endowment for the Arts and the National Endowment for the Humanities.

Through the glass roof from the floors below, the building's clock tower can be seen against the sky. The renovated tower will have a new clock, an observation platform (second only to the Washington Monument in height) and the bells that were Britian's bicentennial gift. The bells have been stored in Baltimore, and the Post Office developers are hopeful that Queen Elizabeth, or at least Prince Charles, will be on hand April 19 for the dedication ceremony.

The commercial space, called the Pavilion, was designed by Boston-based Benjamin Thompson & Associates, architects for Faneuil Hall and Baltimore's Harbor Place.

The Pavilion will be managed under a 55-year lease by Evans Development Co. of Baltimore--a young firm founded by alumni of the Rouse Co. in Columbia, which was one of many companies that competed for the General Services Administration contract.

When asked whether Washington has the customers for the Pavilion's expensive shops and numerous restaurants, President Charles C. G. Evans said research conducted by his firm two years ago indicated that the neighborhood could support such a development.

"We did a small econometric model and a major look at density, flow and pattern," Evans said. "We found that the density was wonderful.

"In our backyard is the Smithsonian. There's no place there to eat. We thought, 'Wow!, what a place to have restaurants.' There are 3,500 hotel rooms going up on the north side of the avenue, the new convention center a few blocks away, about 1 million square feet of new office space planned for downtown. Washington has the largest percentage of of working women of any city, and many of them don't have time to go shopping when they get home at night."

Evans says the the Pavilion will be 60 percent food, 40 percent retail. The mezzanine level will have about 20 food arcades whose customers will sit at tables in front of the stage. There also will be what Evans calls "impulse shops" on this level.

A grand staircase, flanked by cafes, will lead to the next floor, which includes two 6,000-square-foot restaurants, sidewalk cafes outside the building and "slightly higher-priced" stores, says Evans. The third level will have a balcony restaurant and space for more shops.

Evans declined to say who has sublet Pavilion space from his company, preferring to save the news for a big publicity bang in Feburary. However, he does say, "We're about 50 percent committed, and we have active prospects that can fill the remaining space." CAPTION: Picture 1, Old Post Office's inner courtyard, with a skylight view of the renovated bell tower.; Picture 2, Welder works on a steel support for a new balcony in Post Office's 10-story atrium.; Picture 3, Spiral staircase parts at Post Office, whose renovation is expected to cost $20 million.; Picture 4, Ongoing construction in the Post Office's courtyard, where the main attraction will be a stage. Photos by Fred Sweets -- The Washington Post