Expansion of the Japanese chancery on Massachusetts Avenue was approved unanimously this week by the District of Columbia Board of Zoning Adjustment in the first application members considered under emergency zoning regulations passed in April.

The vote clears the way for the Japanese Embassy to start work on the first of a two-phase building program on the three acres of land it owns at 2520 Massachusetts Ave. NW. The addition will allow the Japanese to move all chancery offices from two other Northwest Washington office buildings onto the Massachusetts Avenue property where the Tokyo government built its first diplomatic offices in 1932.

The emergency regulations were voted to allow the Japanese and at least two other governments to proceed with plans to relocate or enlarge their chanceries while city officials completed revisions needed in D.C. zoning regulations. The city must amend the rules to comply with the Foreign Missions Act, which became law last Oct. 1 and requires foreign governments to get State Department permission to buy, sell, rent or expand property in the United States.

The government of Portugal would like to renovate its chancery on Kalorama Road NW, and Saudi Arabia wants to buy the five-story Peoples Life Insurance Co. building near the Watergate.

In presenting the Japanese request, Embassy First Secretary Kyochi Komachi said that "of our 185 missions worldwide, Japan considers the mission in Washington is the most important." He also said that when U.S. diplomats asked for permission to build an 11-story chancery in Tokyo "construction started within a month after the application was completed."

A report by traffic consultant Robert L. Morris said "the increase in traffic that would result from the expansion would be imperceptible." In addition, the 250 parking spaces the Japanese plan to provide in underground parking are more than enough to serve parking needs for years to come, according to Morris and the Japanese officials.

Two community groups--the Sheridan-Kalorama Neighborhood Council and Advisory Neighborhood Commission 1-D--sent letters approving the Japanese plans for "retaining the historical aspects" of its buildings and the promise to "keep this disruption during construction to a minimum. . . . "

Even opposition to the expansion plan was muted. Charles R. Burrows, who lives near the Japanese chancery, praised the "tastefulness" of the planned additions but urged the zoning adjustment board to take into account the kind of precedent that could be set by approving the expansion. He said the "embassy row" of Massachusetts Avenue stretching northwest from DuPont Circle is already congested, and that additional traffic will be generated if other embassies expand their offices.

The zoning board voted 5-0 to approve the Japanese plans, but added two conditions. One requires the Japanese government to implement its expansion plans in the form presented at this week's meeting, which call for construction in two stages to minimize disruption in the neighborhood. The other condition asks the Japanese government to limit use of a tunnel entered on Waterside Drive. The tunnel leads into the parking garage and is planned as a "security device" for use by only a few diplomats or visitors, the Japanese said.

The purpose of the Foreign Missions Act is to give the United States leverage in demanding better treatment for U.S. diplomats abroad, said a State Department spokesman. Approval of plans by friendly governments to buy, sell or expand property for embassy use is expected to be given routinely, said the spokesman.