A plan that requires foreign governments to build their offices along major avenues in Washington and not in low density residential neighborhoods was unanimously adopted by the National Capital Planning Commission yesterday.

"It's a milestone." Edward Hromanik, assistant director for comprehensive planning for NCPC, said of the plan. "It's the first time since the L'Enfant plan (of 1971) that there has been a long range for the development of foreign missions and international agencies in the District." The NCPC is the federal government's planning agency for the metropolitan area.

The plan, which has the support of the city and the U.S. State Department, encourages development of chanceries and international agencies along Massachusetts Avenue from the Naval Observatory to Union Station, along 16the Street NW, in the Van Ness Chancery area and the Van Ness Uptown Center near Connecticut Avenue and Van Ness Street, and in the downtown areas of the city. Embassies, which are the residences of ambassadors (as opposed to chanceries, which are the offices for foreign governments) would not be prohibited from residential areas.

And, according to NCPC's plan, the McLean Gardens, apartment complex, at Wisconsin Ave., north of the National Cathedral, is ruled out as a site for chanceries.

Two years ago, CBI Fairmac Corp., the owner of McLean Gardens, announced that it planned to convert the 723-unit complex to an enclave of embassies, chanceries and luxury apartments. Before that, CBI Fairmac had planned to convert the units to condominiums. Both proposals were vigorously opposed by the tenants.

R. Robert Linowes, the attorney for CBI Fairmac, said yesterday that he has not seen the final plan adopted by NCPC, but he doesn't believe it signals the end for Fairmac's plans for McLeans Gardens.

"We don't think anything's over by any means." Linowes said. He noted that CBI Fairmac has a zoning application to convert McLean Gardens into chanceries and embassies pending before and the D.C. Zoning Commission.

Kirk White, of the city's Municipal Planning Office, said, "It would appear at this point that the zoning commission will not allow chanceries at McLeans Gardens," but he added that the situation will be further reviewed.

David Childs, chairman of the NCPC, said the plan is "significant" because it represents the enactment of the first phase of a comprehensive plan for the city.

The plan adopted yesterday now will go to the D.C. Zoning Commission for implementation. Municipal planning director Ben W. Gilbert said his office will recommend to the zoning commission that a new chancery zone be created to carry out the plan.

Gilbert said his office is "supportive" of the plan adopted yesterday, and both NCPC and city officials said recommendations of residents were considered in adopting the plan.

"When carried out, it will protect our residential areas from being overdeveloped with chanceries and will at the same time provide adequate space for chanceries," Gilbert said. "That was the delicate balance we tried to achieve." He said the plan recognizes the Embassy Row areas on Massachusetts Avenue and 16th Street and also will allow chanceries in developed commercial areas.

According to an NCPC report, Washington now has 130 foreign missions and 17 international agencies, wich employ 16,300 people. The report estimates that by 1995, there may be between 140 and 176 foreigns missions and 21 international agencies in the city, employing thousands more people.

About 95 per cent of the foreign missions now are west of 15th Streets NW, particularly along. Massachusetts Avenue, the report said. About two-thirds of the international agencies are clustered in the downtown office area west of 17th Street.

Most of the foreign missions have sought new or improved facilities and have moved since 1965, the report continued, and finding suitable and adequate locations has been a "continuously growing problem." Some chanceries have been making inroads into residential areas, and many homeowners oppose them because of the traffic and parking problems associated with office buildings.

Hromanik said construction of three new major projects are not prevented by the NCPC plan because those foreign missions have applications under consideration by the zoning commission. Those projects are the proposed French Chancery on Reservoir Road, the Italian Chancery on Albemarle Street. NW and the World Bank on 19th Street NW.

NCPC plan states that it shall apply immediately to all foreign missions and international agency facilities except those for which an application has been filed with, and a public hearing granted by the zoning commission. In the McLean Gardens instance, no hearing has yet been granted.