The National Capital Planning Commission voted yesterday against a plan to enclose the observation deck at National Airport and convert it to a minimall of nine specialty retail shops.

The Federal Aviation Administration proposed the $600,000 project last year to improve the number and quality of retail services available to commuters at the federally owned airport in Arlington. The shops would double the retail facilities there now.

"If you go to National Airport and you've forgotten your shirt, you can't buy one there," said Bob Crews, owner of Benjamin Book Inc. at the airport and a supporter of the minimall. "You couldn't buy your mother anything but a battery-operated import there."

The enclosure of the 190-foot observation deck outside the main terminal would be done as part of the Department of Transportation's minority business program, FAA spokesman Dave Hess said.

But members of the NCPC, the federal planning agency charged with protecting the federal interest in the area, voted 4 to 1 with one abstention against the plan, saying any improvements to National should be limited to those essential for safety or maintenance.

Their objections also were tied to an NCPC policy that individual projects should not be undertaken in the absence of a master plan. The first phase in the development of a master plan for National has been completed. But it has not been released because of legislation pending in Congress that would transfer control of National and Dulles International airports to a local authority.

Hess said that the NCPC vote was strictly an advisory one, and would not necessarily end the project.

The vote did not come as a surprise to some mall supporters at the meeting because NCPC has long advocated a phased closing of National.

In other action, the commission unanimously approved scaled-down plans for a National Holocaust Memorial near the Mall at 14th Street and Independence Avenue SW.

Architect George Notter said floor space had been reduced from 320,000 to 275,000 square feet, and the front of the building dropped from 82 to 64 feet. The changes would make the memorial more compatible with the Auditors Building and the Bureau of Engraving and Printing that will flank the structure, he said.