The proposed three-sided expansion of the Dulles International Airport terminal could "emasculate and . . . destroy" what architects consider one of the most magnificent buildings in America, according to some concerned members of the National Capital Planning Commission.

The proposed future additions, which ultimately would more than double the size of the passenger terminal, include a previously unpublicized plan to add another two-story, 50-foot extension on the runway side of the long, gull-winged building.

NCPC member and architect R. Randall Vosbeck raised concerns about the additions at a recent commission meeting. "It bothers me that this monument will be destroyed" by the additions, Vosbeck said, particularly the 50-foot additions never envisioned by Dulle's world-renowned architect Eero Saarinen.

"We've got 50-foot additions here and 50-foot additions there and sooner or later by adding on the sides and both ends we no longer will have the same building . .fs. I'm not sure we can add onto it without emasculating it," Vosbeck told the federal planning agency for the Washington area. Last year Dulles was voted one of the three greatest buildings in the nation, according to a poll taken by the Journal of American Architects.

One 50-foot addition to the terminal, where future mobile lounges will dock instead of nuzzling up to the main building as they do now, was approved by NCPC in 1972 and given final approval last March, although several commission members expressed misgivings about it. Construction bids for that addition now are being received by the Federal Aviation Administration, which owns and operates Dulles and National airports.

In addition, the consulting firm now finishing a revised master plan for the airport, to guide its growth until 1995 and beyond, is proposing that this new passenger and baggage loading area be doubled in size, extending it out 100 feet from Saarinen's building.

It also is proposing a linear extension of the terminal, which Saarinen himself had predicted someday would be necessary and which NCPC approved in 1972. The FAA has not built the extension because federal funds for it are lacking, and there are still an insufficient number of passengers at Dulles to warrant it. The length of the present terminal, under the proposed master plan, would be increased from its present 600 feet to almost 1,400 feet, with the extension identical to the existing Saarinen building.

The opposition of Vosbeck and several other commission members surprised FAA officials at last week's meeting. "I suppose it's a delayed reaction . . . a late-blooming concern to the issues we discussed last spring," said James Wilding deputy director of Metropolitan Washington Airports. "We thought we were already over that hurdle."

The commission has set aside a half day at its January meeting to reconsider the final draft of the Dulles master plan. While the commission's opinion is merely advisory, the FAA may disregard an NCPC disapproval only if it gives detailed reasons in writing for doing so.

Commission members also were concerned that the consultant's briefing they were given last week on the proposed new master plan by the airport-planning firm of Peat, Marwick, Mitchell & Co. did not clearly show how the 100-foot addition the firm was proposing would look. The only slide showing the architectural model of the building was photographed from the side opposite the addition.

The NCPC concern over Dulles raised the decade-old issue of whether the airport terminal, because of its architectural merit, should be placed on the National Register of Historic Places, as long urged by many architectural groups and the Department of Interior, custodian of the register and its 14,000 historic buildings and places across the nation.

The FAA has opposed Dulles's inclusion on the National Register, because, among other reasons, it felt the process of running and making changes at a working airport might be delayed and become unduly cumbersome. However, Wilding told the commission last week it is now reviewing its opposition and will meet shortly with the Advisory Council on Historic Preservation, which reviews changes proposed for any place on the Register.

If Dulles were placed on the Register it would be one of the youngest buildings on it. It was completed in 1962. Several 1930s buildings and the 1949 Commonwealth Building in Portland, Oregon, by architect Pietro Belluschi, currently are among the most recent buildings given Register protection. While the Advisory Council's opinion, like NCPC's, is only advisory and cannot stop a federal agency from destroying or changing an historic building, it provides an aesthetic review and focuses national attention on threats to America's architectural landmarks.

The other aspects of the proposed Dulles master plan drew little comment from the planning commission, but some parts of it have brought objections from Fairfax County officials. The county has asked that an environmental impact statement be prepared on the plan's proposed acquistion of up to 3,000 more acres around the airport both for additional runways and as noise buffers. The consultants have considered only the internal problems of the airport and not the effects on the county, Fairfax officials complained two weeks ago. Dulles straddles the Fairfax and Loudoun County border.