The Metropolitan Washington Airports Authority moved yesterday to promptly finish planning for a new midfield terminal at Dulles International Airport, clearing the way for construction to begin as early as next spring.

In voting to spend almost $3 million to finish design work for the new terminal and other major improvements, the airport board essentially endorsed plans for the expansion of Dulles that had been devised under the Federal Aviation Administration before it turned over the operation of Dulles and National airports two months ago.

Yesterday's vote was significant, authority members said, because there had been speculation that the new board might want to replace the FAA's vision for Dulles with radically different plans of its own.

The chairman of the authority, former Virginia governor Linwood Holton Jr., emphasized yesterday that the board retains authority to reshape Dulles' future as it sees fit. But he said the authority has enough confidence in the FAA's plans to move ahead with the first phase of Dulles expansion.

Those plans include a new 26-gate, midfield terminal, an underground railway linking the new facility with Dulles' main terminal, and a new international arrival area, not far from its current location in the main terminal.

About 60 percent of the planning work for these improvements had been completed before the airport transfer took place, said James A. Wilding, general manager of the airport authority. The remainder of the work by the airport board's architects -- Skidmore, Owings, and Merrill -- will cost between $2 million and $3 million, Wilding said.

If design work, construction bids and financing -- using the authority's legal authority to sell bonds -- are completed promptly, work on the improvements can begin at the start of the 1988 construction season, Holton said. Other board members cautioned that this scenario may be too ambitious.

Long-term plans for Dulles call for as many as six midfield terminals, as well as new runways and parking structures. Some airlines are now operating from temporary terminals on the midfield.

The facilities expansion comes as the airport's phenomenal growth in flights and passengers continues unabated; 11.1 million passengers used Dulles during a 12-month period ending in June, up 64 percent from the previous year, officials said.

In other action at yesterday's meeting, the authority voted to reduce penalties for various violations by taxicab drivers, hoping that lower penalties will lead to more rigorous enforcement.

Currently, some violations, such as ignoring a taxi dispatcher, are Class 1 misdemeanors, punishable by up to a year in prison and a $1,000 fine. Because these violations often result in time-consuming trials, Wilding said, enforcement has been lax.

Beginning Sept. 6, the violations will be Class 4 misdemeanors, with fines that will be payable by mail. At the insistence of Arlington County authorities and some taxi drivers, Wilding said, the authority scrapped a proposal to lower the penalty for driving a cab without a taxi license.