Finishing the Federal Triangle is Washington's oldest unfinished business.

Architect Harry Weese, who won a General Services Administration competition to design a plan to finish the last major section of the federal city, presented six different plans at a press conference at the National Gallery of Art's East Building yesterday. Weese and his staff designed Washington's Metro system.

The major attention was on the 12-acre Great Plaza behind the District Building and the area around the Old Post Office.

Jerry Shiplett, chief of GSA capital facilities planning, said GSA hopes to have the new Federal Triangle plan finished and approved by all interested agencies by late next summer. GSA will ask Congress to approve some funds by fiscal 1983 or 1984.

The Great Plaza plans range from minimal expenditure to a maximum development costing in excess of $100 million. The redevelopment area around the Old Post Office would cost a projected $21 million.

Under general consideration is the area between Pennsylvania and Constitution avenues, 15th Street to Sixth Street. The plan includes opening up to the public most of the now-restricted courtyards and embellishing the ground floors of the federal office buildings, including Commerce, Labor, Internal Revenue, Justice and the New Post Office Building. The Old Post Office Building is already being remodeled for offices and retail space by an Arthur Cotton Moore design.

The Weese plans are all part of the intensive development of the Pennsylvania Avenue inaugural route, by both private and federal interests, overseen by the Pennsylvania Avenue Development Corporation.

Development of the huge hidden 12 acres behind the District Building, now used as a 2,500-car parking lot, is expected to be controversial.Weese's six alternatives begin with a minimal development with trees and a reflecting pool, headed by the existing Strauss fountain.

At the other extreme, Weese suggests four new eight-story buildings -- federal offices above, information and exhibit space on the ground floor and commercial space and parking below ground.

"The area is enormous," Weese said. "You could put several buildings the size of the National Gallery of Art's East Building in the area." A ceremonial pedestrian parade -- "avenue is a dirty word," Weese said -- would slice through the buildings from Pennsylvania Avenue to 14th Street, aiming at the Washington monument.

"It would link the commercial city to the Federal city. It would be a bridge between the two. Now the Federal Triangle is a barrier. This passageway would be a link. Instead of keeping people out of their government buildings, this would allow them to go through them," he said.

The other four schemes are variations on these two, but all provide a new building where the Old Coast Guard building used to stand, east of the District Building. In some versions, this building would be a part of the New Post Office Building.

In all the Weese plans, the curving Internal Revenue Building would sprout a new limestone facade, festooned with vines, about 20 feet out from the current front. The arcade would face on a forgotten plaza now used as a 200-car parking lot. Weese hopes it would become a "World's Fair of Restaurants," with small, ethnic cafes, "presenting the culture of foreign countries to America."

Offices would go on the upper floors of the new extension. All surface parking would be removed, but some would go underground below the plaza. The plaza at ground level would have a removable stage for speeches and other events. The ends of the IRS building, left uncompleted for years, would be finished in high Federal Triangle style with columns and such.

On 12th Street, the hemicycle between the New Post Office Building and the Old Post Office building was once to be completed as a circle when the Old Post Office was torn down. Weese's plan would leave it as a plaza with some sort of water feature, and a fancy pavement dipping down to provide an easy access entrance into the South entrance of the old building.