The General Services Administration wants to spend $6.1 million to modernize Washington's only "tempo" building remaining from either World War I or II -- the Liberty Loan Building beside the Tidal Basin.

The porject is opposed by the staff of the National Capital Planning Commission, who say it would waste money and perpetuate an eyesore beside the Tidal Basin and the Mall at the major entrance to the Nation's Capital. The commission votes today on the proposed renovation plan.

The five-story building is the first structure one sees upon entering Washington across 14th Street Bridge, and except for the Jefferson Memorial, it is the closest building to the Tidal Basin. Its loading docks, trash "dumpsters" and parking lots face the Tidal Basin, less than 100 yards away.

The 1918 building, built on parkland, will need to be virtually gutted inside, according to a GSA report.All its wooden windows are rotten, the plumbing, electricity, telephone lines, ceilings, walls, stairs and elevators must be replaced, as well as the exterior stucco on the building. Air conditioning, insulation, fire sprinklers, rest rooms and new floor coverings would need to be installed.

The cost of the repairs, seven times the original cost of the building, would be less expensive than constructing or leasing a new building for the 504 Treasury Department employes who work there, according to GSA.

The current estimate of $6.1 million is nearly double the $3.5 million estimated last year by GSA for the cost of modernization. Although GSA presented figures to Congress to justify last year's estimate, Congress has yet to appropriate any funds for the renovation.

Under all long-range master plans for the Mall area, the Liberty Loan Building would eventually be demolished and the site returned to the parkland it was before World War I.

In reviewing GSA's 1980 capital budget last year, the planning commission recommended against the Liberty Loan renovation. Today, however, is the first time the project officially has come before the Washington area's federal planning agency.

The NCPC staff says the escalating cost estimates justify a reevaluation of the renovation "from the standpoint of cost alone."

But the main reason is should be opposed by the planning commission, the staff says, is that the temporary building "is out of character with the family of great landmark buildings" around the Mall and Tidal Basin and detracts from the major gateway to the Nation's Capital.

The building originally was constructed as a "temporary" three-story office for the World War I Liberty Loan Bond drive. Two floors were added to the L-shaped building in 1927, and the auto ramp onto 14th Street from Maine Avenue was cut through the ground floor of the building in 1952.

Most of Washington's remaining "tempo" buildings from both World Wars were demolished in the '60s, including several along the Mall where the Smithsonian National Air and Space Museum is now located.

Six blocks of World War I Navy Department tempos on Constitution Avenue between 17th and 23rd Streets NW, about 100 yards from the Reflecting Pool, were torn down in 1970 and Constitution Gardens created in their place. President Franklin D. Roosevelt, then under-secretary of the Navy, ordered their construction there in 1918 but later said he regretted allowing them to be built so close to the Mall.

The federal Commission of Fine Arts, founded in 1910 to help guide the architectural development of Washington, has not opposed the GAS plan to renovate the Liberty Loan building, although one member at last month's meeting joked that if this last remaining World War I "tempo" is renovated, someone soon may want to have it declared an historic landmark and preserved in perpetuity.