Congress approved $24.1 million in improvements yesterday to two federal buildings in the Washington area, including the historic old Post Office Building in the downtown Federal Triangle, a structure that was once marked for demolition.

It authorized $18 million to convert Street and Pennsylvania Avenue NW, a 78-year-old landmark topped by a lofty clock tower, into the clock tower, into the cornerstone of the Pennsylvania Avenue redevelopment Program.

The decision wound up more than a decade of efforts by preservationists to retain the structure.

Congress also approved $6.1 million to repair and alter the sprawling warehouse operated by the General Services Administration (GSA) near Franconia, in Fairfax County. The structure, built in 1953, has 1 million square feet of floor space on a 24-acre site south of Springfield Mall shopping center.

Approval came yesterday when the House Public Works and Transportation Committee approved the formal proposals, each called a "prospecturs," submitted by GSA, official landlord of most government buildings. The Senate Environment and Public Works Committee had approved both projects earlier.

Under federal law, most federal building construction projects are approved whtn the prospectuses are approved by two committees. Actions by the full House and Senate are not required.

The Post Office Building project represents a departure from the past. GSA has chosen a plan by Washington architect Arthur Cotton Moore to convert the building into a sort of "bridge" between the cityscape to the north of Pennsylvania Avenue and the federal enclave to the south.

On lower levels, surrounding the open, glassed-over courtyard area of the remodeled structure, will be shops and a restaurant. Upper levels would be occupied by offices. The clock tower would become a public observation platform.

According to the GSA prospectus, much of the cost of renovation would involve replacement of the building's obsolete electrical, heating, air-conditioning and mechanical systems, replacement of the roof and installation of modern elevators and a fire-protection system.

Foundation, some vulnerable to decay by nearby Metro construction, would be restored, the GSA prospecturs said.

The biggest item in the Franconia project is a new roof, to cost $3.7 million. GSA said wooden roof trusses on the flat-topped structure have failed because of heavy winter snow.

Repair of the old structure is far more economical than building a new structure at a cost of up to $30 million, the GSA prospectus said.

In addition to the two building reovations, Congress also approved a 10-year lease renewal at an estimated $850,000 annual rent for the privately, owned building at 1441 L St. NW occupied by the Small Business Administration.