A consultants' report released yesterday recommends a major expansion of Dulles International Airport, including the previously proposed midfield passenger terminal, an addition to the existing terminal, an underground "people mover," and new roadways, parking lots and runways.

All could be needed to accommodate increasing air traffic at Dulles expected through the year 2000, the report said.

The 153-page report by three consulting companies recommends various actions based on different growth rates projected for Dulles, which has been chronically underused since its opening in 1962. The most ambitious plan would cost $343 million, according to the study.

Dulles spokesman David Hess said that figure is "purely conjecture," and noted that the Federal Aviation Administration, the airport's owner, has not decided what projects should be built.

The report, the first master plan undertaken for Dulles since the 1950s, elaborates on an earlier consultants' report released in March that recommended construction of a midfield terminal about 1,200 feet south of the existing terminal. Airline officials, local officials and Dulles proponents say the proposed terminal and other improvements, including the added runways, are crucial to Dulles's growth.

"It's absolutely critical for the continued growth of the airport," said Rep. Frank R. Wolf (R-Va.), a Dulles promoter. "Without it, Dulles won't be able to make its required growth."

The report, by Peat, Marwick, Mitchell & Co. and two other consulting companies, recommends that the FAA buy 1,100 acres of land adjacent to Dulles to make room for two runways in the event they are needed. The airport now has three runways, and the other two would give Dulles the runway capacity of the nation's largest airports.

"Airlines are very interested in runway capacity, because one of an airline's biggest problems is being able to get your passengers up and down on time," said Thomas Morr, head of the Washington Dulles Task Force, a group that promotes the airport.

The study also said the FAA may have to expand the current terminal, designed by Finnish architect Eero Saarinen, from its current 920 feet in length to 1,200 feet. About 3.4 million passengers pass through Dulles each year, and the number is growing by 17 percent a year, FAA officials said. But the lack of the new terminal and other facilities has discouraged airlines from scheduling flights there, they said.

Since the airlines were deregulated in 1978, airlines have routed more of their flights to "hub" airports, where an airline tries to schedule connecting flights for its passengers. Dulles officials have tried for years to persuade airlines to locate hub connecting flights at Dulles.