The National Capital Planning Commission yesterday approved construction of a $50 million underground complex on the edge of the Mall to house two new Smithsonian Institution museums -- one for African art and one for Eastern art.

The subterranean building just south of the present Smithsonian castle on Independence Avenue SW would become the new home of the Museum of African Art currently housed in the Frederick Douglass house on Capitol Hill and in several adjacent town houses. The Eastern art portion of the complex would be an extension of the nearby Freer Gallery and hold various oriental collections.

Except for funding and approval of the exact design of the building, yesterday's planning commission action gives the Smithsonian the final governmental blessing needed to proceed with the project.

The Smithsonian already has pledges of $5 million from foreign nations and will ask Congress this spring for authorization and $25 million to build the museum. The Smithsonian expects to raise the remaining $20 million from private sources.

More than 95 percent of the new 460,000-square-foot museum will be built below ground with two small above-ground pavilions to serve as entrances. It also will house an underground public parking lot and office space for other Smithsonian activities.

The four-acre site, now a formal Victorian garden and an employe parking lot, is bordered by Independence Avenue and the Smithsonian's three most historic buildings -- the original 1855 castle, the Arts and Industries Building, built in 1881, and the Freer Gallery of Art, built in 1923.

While approving the location of the new museum complex, the federal planning agency deferred for further study the controversy about whether the complex should temporarily or permanently house the African arts museum.

The NCPC staff, the Joint Committee on Landmarks and the city's prestigious Committee of 100 on the Federal City had strongly recommended against putting the African Museum underground near the Mall because, they argued, it would not give the museum sufficient prominence.

All urged the Smithsonian to consider a downtown location near the Gallery Place Metro subway station where the Smithsonian already has two major museums, the National Portrait Gallery and National Museum of American Art (formerly National Collection of Fine Arts) and where numerous private art galleries also are situated.

But Warren Robbins, who established the African Museum in 1964 and has been its director since the Smithsonian acquired his African collection in 1979, warned the planning commission yesterday that if it did not approve the proposed underground museum now, "We stand to lose it.

"You would be selling the museum down the river," he said, because two years of planning, gifts and collections pledged for it and the momentum to build the museum could be lost.

Robbins and Smithsonian officials oppose a downtown site because, Robbins said, "it belongs on the Mall, symbolically and because there it could attract not just 100,000 visitors a year but 1 to 2 million."

Smithsonian Secretary S. Dillon Ripley and several nationally prominent architects also spoke for the project. Ripley said that, while the Smithsonian as an excellent site for the African museum, it might be moved to an even larger building in the future as the African collection grows.

"Remember that the castle once housed the entire museum and staff, some of whom lived there," Ripley said. "Other collections were once housed in freight cars on the Mall till the Arts and Industries Building was built, and what is now the National Air and Space Museum was housed in quonset huts."

The specific detailed design of the underground building and the pavilions and garden that will go above it still must be approved by the planning commission as well as the Fine Arts Commission and the Joint Committee on Landmarks. Officials say they expect no major obstacles.

Yesterday's planning commission vote called on the Smithsonian to give further study to reducing the size of the pavilions and two walls proposed to go along Independence Avenue, relocating the underground parking garage entrance and studying the problem of growing large trees on top of an underground building.

Commission members said they were concerned that the large pavilions and walls would act as barriers, blocking the view of the historic Smithsonian buildings from Independence Avenue.