The General Services Administration is planning to complete the long-unfinished Federal Triangle by building two mammoth office buildings to house 9,000 government workers on what is now a huge parking lot behind the District Building at 14th Street and Pennsylvania Avenue NW.

GSA has decided to throw out plans drawn at the turn of the century that called for constructing a "Great Plaza" facing the 14th Street entrance to the Commerce Department.

Instead, the government's housekeeping department wants to cover the plaza site with a 10-story office building wrapping around a vast, glass-roofed "Galleria" and to put up a 10-story office structure next to the District Building.

The massive construction project so far has drawn little attention outside the federal bureaucracy. Details of GSA's plans are disclosed in the first draft of an environmental impact statement for the complex.

The buildings GSA wants would cost an estimated $350 million at today's prices and would be three-fourths the size of the nearby FBI headquarters.

The new complex would increase by 50 percent--to 27,000--the number of government workers in the triangle formed by Pennsylvania Avenue, Constitution Avenue and 15th Streets NW.

The Federal Triangle was created by Pierre l'Enfant's original plan for the District of Columbia. When a commission of the nation's leading architects updated the city plan in 1901, the triangle was designed as the principal federal office complex, setting the architectural pattern for a century of government growth.

Now GSA, with the aid of Harry Weese & Associates architects, is preparing a new master plan for the triangle to fill in the one gap remaining after 80 years.

When hearings were held on the environmental impact of the project last summer, nobody showed up, said Jerry Shiplett, chief facilities planner for the regional GSA office.

Shiplett said GSA plans to ask Congress to authorize the Federal Triangle completion this year and to seek an appropriation to build it in the fiscal 1984 budget.

GSA officials contend the multimillion-dollar building would save taxpayers money by enabling several thousand workers now housed in rented offices to be moved into government-owned quarters.

No decision has been made yet on what agencies would occupy the new buildings, Shiplett said. Because the location is one of the choicest in Washington, the space probably will be assigned to cabinet-level agencies and government operations with extensive public contact.

The Justice Department headquarters two blocks away already has asked for space, he said. The nearby National Archives is interested in moving its "roots collection" of genealogical records to the new building.

Museum and exhibition space and spots for several privately operated retail shops are included in the proposed GSA "Galleria" building. Facing the 14th Street entrance to the Commerce Department and the neglected Strauss Fountain, the new building would be split down the middle by a glass-topped atrium or gallery.

The "new 10-story structure would fill most of the Great Plaza," GSA's project report says, and would create new, small plazas adjacent to the District Building, Customs Building and the curving facade of the Federal Building, known as the new Post Office.

A second 10-story building would be built just east of the District Building facing Pennsylvania Avenue and connected to the existing Federal Building.

The Federal Building now has a raw brick wall on its west side where the original plans called for building another wing. That wing never was finished because funds for Federal Triangle construction ran out during the Depression.

The original plans also called for new buildings on the site of the District Building and the Old Post Office between 11th and 12th streets on Pennsylvania Avenue.

Historic preservation groups rescued the Old Post Office from wreckers a decade ago, and now GSA is completing renovation of that building into an office, restaurant and entertainment complex.

Plans to move the D.C. government out of the Federal Triangle were scrapped long ago, leaving only the huge L-shaped parking lot to build on.

The environmental impact statement for the Federal Triangle Master Plan discloses GSA studied a dozen different uses for the site and decided to build the biggest buildings possible.

The agency rejected a "minimum build" proposal that called for filling in the gap between the District Building and the new Post Office with a seven-story office building and replacing the parking lot with a park. That alternative would have permitted building the Grand Plaza much as it was planned at the turn of the century, with rows of trees flanking a long reflecting pool.

Also turned down was a "medium development" scheme with one 10-story building facing Pennsylvania Avenue, another 10-story building facing 14th Street and a smaller plaza.

Compared to the 263,000 to 375,000 square feet of space in the smallest plan and about 768,000 feet in the medium-sized one, the GSA proposal calls for between 1.6 million and 1.725 million square feet of offices, another 70,000 square feet of retail and exhibit space and parking for 2,100 cars in underground garages.

All of the alternatives considered by GSA call for replacing the present parking lot with underground parking.

No architects have been chosen to design the building, but GSA has spelled out in some detail the features that will have to be incorporated into the final blueprints. The building will have direct access to the Federal Triangle Metrorail station and a glass-topped core running east and west to handle pedestrian traffic.

The National Capital Planning Commission will have to approve construction of the Federal Triangle building and the D.C. Commission on Fine Arts will review the design. Both agencies have been briefed on the project, but neither has taken any action.