United Airlines, the nation's largest, announced yesterday a major Washington-based East Coast expansion that will immediately make it the largest single carrier at suddenly flourishing Dulles International Airport.

James J. Hartigan, United's president and chief executive officer, said in a news conference here that starting next May, Dulles will be a new "hub" for United, with more than 50 daily departures to 24 destinations. "Our plan is to increase our daily service to more than 100 daily departures within three years," he said.

The announcement means that United will throw its enormous financial and scheduling strength at such new-era airlines as Presidential Airways and New York Air, already engaged in a vigorous fare war at Dulles that could intensify, to the benefit of consumers.

It also will be another boost for Dulles, which lay moribund on the Fairfax-Loudoun county line for 20 years, but suddenly has become the hottest airport ticket in the East because there is still room.

In October, Dulles handled 530,000 passengers, a 63 percent jump from the same month a year earlier. For the 12-month period ended in October, Dulles had 4.7 million passengers, up 36.5 percent from the previous 12 months. Commercial and residential development around Dulles has been strong and turned area roads, particularly Virginia Rte. 28, into rush-hour parking lots.

That growth is part of what attracted United, Hartigan said. "The area is an important place in itself, there is a large local market," he said. In addition to the bedroom communities surrounding Dulles, he cited "some very important, key commercial developments."

United could become the largest single airline serving the Washington area. New York Air has 74 flights from Washington today, 43 at Dulles and 31 at National. Eastern Air Lines is the biggest carrier at National, with 54 flights a day. United has 16 flights a day at Dulles, mostly to Denver and the West Coast, and 17 from National, mostly to Chicago.

To handle its Dulles passengers, United will build a temporary terminal on top of its present maintenance facility in the middle of the airport. The terminal will have 10 or 11 gates, Hartigan said. Passengers will be shuttled to the terminal by mobile lounge from the main Dulles building and will be able to connect with all other United flights at the terminal.

That concept was introduced at Dulles by New York Air, which has its own temporary terminal at the base of the tower. Presidential plans to open a new 15-gate temporary terminal at Dulles later this month.

Geoffrey Crowley, Presidential's senior vice president for marketing, said that he is not afraid of United's plans and marketing power. "I see this as really establishing Dulles as the airport in Washington," he said. "If people have a choice of us or United, a major number will choose us."

Clark Onstad, a vice president of Texas Air Corp., which owns New York Air and Continental Airlines, said, "We have a long history of competing successfully with United at Denver, and look forward to the opportunity to compete with them at Dulles."

"We will be competitive," Hartigan said. "We wouldn't undertake a venture like this and put our capital and our resources at risk if we felt we could not only be competitive, but be successfully competitive."

The Dulles boom has come while Congress is debating legislation to transfer control of Dulles and National from the Federal Aviation Administration to a regional authority. The airlines support that move partly because the two airports could sell bonds to finance improvements, something the FAA cannot do.

The FAA wants to build a big midfield terminal at Dulles to replace the temporary facilities now planned or in use, but "we can't actually build the midfield terminal" until the airport authority is created, said James A. Wilding, the FAA's director of Metropolitan Washington Airports.

United is primarily an east-west airline and does not participate in north-south traffic, but with this move will do so. Hartigan said United flights from Dulles will serve Boston; Hartford, Conn.; New York's LaGuardia; Newark; Rochester, N.Y.; Cleveland; Buffalo; Tampa, Fort Lauderdale and Orlando, Fla; Raleigh-Durham, N.C., and New Orleans, and other cities. All those destinations are served by New York Air or Presidential, or both.

United chose Dulles over a southwestern hub, Hartigan said, because the potential increase in traffic looked from "five to eight times bigger" and would not divert passengers from United's present strongholds at Chicago and Denver.

The United move must also be seen as part of its continuing strategy to keep up with the competition. American Airlines, also an east-west carrier, is expanding into the north-south market with new hubs at Nashville and Raleigh-Durham.

This year, United has taken a 29-day pilots' strike to win a major cost-saving contract; has spent $750 million to buy airplanes and the Pacific routes from Pan American World Airways; placed a $3 billion order with Boeing for up to 116 new airplanes, and purchased Hertz from RCA Corp. for $587.5 million.

John L. Cowan, United's senior vice president for administration and finance, said in an interview in May, "Once we get the job done on the pilot wage side, we can expand this airline very, very fast."