United Air Lines announced plans yesterday to add flights and $40 million in improvements to Dulles International Airport over the next two years, underscoring its expectations for continued economic growth in the surrounding area.

"We think this is the hottest development corridor on the East Coast," said John Zeeman, United's executive vice president of marketing and development. "It's a major success story."

Zeeman's remarks came a week after Fairfax County voters rejected the public official most associated with the area's building boom, Board of Supervisors Chairman John F. Herrity, voting to replace him in January with the county's most vocal critic of his progrowth policies, Annandale District Supervisor Audrey Moore.

The announcement also came one week after Dulles airport reported an 8.9 percent decrease in passengers in September compared with September 1986, reflecting in part the decision of Continental Airlines to scale back operations there.

"United believes this will be the fastest-growing area in the East," Zeeman said at a meeting of the Washington Airports Task Force, a private group that promotes Dulles and National airports. "This area will have a very, very strong local market."

Task force members applauded Herrity as he entered the meeting, representing Fairfax County, and they expressed thanks for his efforts to promote Dulles over the years.

The economic development that has clogged local roads and fueled the election debate in Fairfax County also has flooded Dulles' parking, baggage claim, ticketing and waiting areas with passengers.

When it opened in November 1962, Dulles was designed to serve up to 4 million passengers a year. The airport, which celebrates its 25th anniversary next week, served more than 11 million in the year that ended Sept. 30.

While many additions have been made, Dulles still needs more than $500 million in improvements to its parking, terminal, baggage, international arrival and airfield facilities, said Carrington Williams, chairman of the task force and a member of the local authority created to finance about $1 billion in construction at Dulles and National.

United plans to support the Metropolitan Washington Airports Authority in "all phases of its development program at Dulles," including the plan for a new midfield terminal linked by underground train to the main terminal, Zeeman said.

United "is prepared to be the anchor tenant" in the new midfield terminal, subject to a formal agreement, Zeeman said, giving a major endorsement to a proposal that has been criticized as too costly by officials of Texas Air Corp., a major competitor of United at Dulles and Denver's Stapleton International Airport.

Continental, which is owned by Texas Air, decided in July to cut more than a third of its flights at Dulles, primarily to consolidate the operations it absorbed through recent mergers with New York Air, People Express and Frontier Airlines.

The People Express acquisition tied Continental to using Newark as a hub, competing with Dulles as a north-south connection point, airline industry experts say. Improvements at Dulles could make Newark less attractive to travelers needing connecting flights, they say.

Zeeman said United has spent more than $20 million in the past two years to upgrade gates, baggage, ticketing and other facilities at Dulles, and it plans to spend $40 million over the next two years.

United, already the dominant carrier at Dulles, plans to increase its flights there from the current level of 87 a day to more than 100 by the end of next year and to more than 200 "eventually," Zeeman said.

The airline said in July that it would offer 110 flights a day at Dulles by the end of this year, but it is delaying the additions because of aircraft sales and purchases, an official said.

In other action, task force members voted to change the organization's name to the Washington Airports Task Force, from the Washington Dulles Task Force.