The National Capital Planning Commission has approved the expansion of the new International Center on Connecticut Avenue NW to allow an additional nine nations to build embassies or embassy offices on the hilly, wooded federal land west of the Van Ness Metro station.

The federal planning agency also approved final plans on part of the site for the $50 million headquarters of the International Telecommunications Satellite Organization. The 106-nation cooperative agency, among other things, coordinates international satellite launchings and earth orbit positions.

An ultramodern glass and stainless steel structure, the Intelsat building will be terraced on the side of the hill above Connecticut Avenue and Tilden Street in a series of nine four-story "pods." The design has been praised by architectural critics as perhaps the most advanced and original office building yet seen in the Washington area.

The Intelsat building and the expansion of the International Center require final congressional approval, which State Department officials expect will come within the next few weeks. Intelsat's 472 employes currently work out of several office buildings at L'Enfant Plaza.

The International Center, the first officially designated diplomatic enclave in Washington, ultimately will cover 47 of the 69 acres of the old National Bureau of Standards-- which moved to a $120 million headquarters in Gaithersburg in the mid-1960s. The University of the District of Columbia is completing its new campus on 22 acres of the site.

Under the master plan for the International Center approved by the planning commission, a total of 23 embassies or chanceries, as embassy offices are called, could be built on one-acre sites. Israel recently completed the first building, a new embassy, and Bahrain and Kuwait are now completing new chanceries. Four other nations also have been leased sites. About six acres of woods will be retained and landscaped as a buffer on neighborhood streets.

The Intelsat building, which is expected to be completed in just over two years, will have an additional four acres in parks on its 12-acre site.

Most area residents apparently favor the embassy enclave and the Intelsat building; few people have spoken against it at public hearings.

"It's a question of alternatives," says Joseph Mintzes, of 3530 Yuma St. NW, one of half a dozen residents whose homes abut the old Bureau of Standards site and back on the expanded section of the International Center.

"If we don't get an embassy-chancery type of use we could get more intense development," said Mintzes, who spoke at a recent hearing on behalf of the Yuma Street Homeowners Association.

The association, and many other neighbors, want more than the proposed 15-foot buffer zone between residents' back yards and embassy grounds. The planning commission agrees and has strongly recommended a minimum 30-foot landscaped buffer. The commission, which in effect has zoning control over the federal property's development, still must approve detailed plans for the expanded center and the individual embassy-chancery buildings.

Sen. Charles McC. Mathias (R-Md.), who heads the Senate District Committee, has questioned whether it might be better to drop the Intelsat building and further expand the embassy enclave, since the State Department has been complaining about the difficulty of finding adequate chancery space here for foreign governments. But federal and city planning officials, the State Department and apparently the House and Senate public works committees oppose such a change.

"Everybody has said since 1968 that this section, on Connecticut Avenue near the subway station, must be high density," State Department lawyer Harold Burman said last week. An office building can be "stepped up the steep slope" but few individual chancery buildings could be built on it, and then only at great expense, he said. "GSA the General Services Administration could put an ordinary office building on the site," said Burman, but it probably would be much larger and could not compare in design with what Intelsat is proposing.

Under the commercial zoning that governs most land around the Van Ness Metro station, an office building for 3,500 employes with 1,177 parking spaces could be built on the Intelsat site alone, according to a planning commission study. The Intelsat building initially will house up to 550 employes, with underground parking for 270 cars, although the building may ultimately be expanded for up to 1,300 employes, with parking for 550 cars.

UDC's Van Ness campus, for example, is being expanded from its present maximum capacity of 6,000 students and staff to 7,000 by 1984 and could grow to 13,000 students by 1990, with most at the Van Ness campus.

An estimated 600 employes would work at the nine proposed embassy-chancery sites and about 600 employes will work at the 14 previously approved sites, according to an environmental impact statement prepared for Intelsat.

ANC 3F commissioner Barbara Luchs said last week the ANC has "approved the broad concept of the International Center, but we haven't seen the detailed plans."

One resident, Robert Carter, who has lived opposite the site at 4322 36th St. NW for 21 years, said his family "would like it to be an embassy compound," not because he's a retired State Department official but because the embassies would be quiet neighbors compared to some other possible developments.