Officials of the Reston Land Corp. unveiled plans yesterday for an 85-acre development of hotels, office buildings, shops, restaurants and residential buildings that they said would become the heart of Reston's long-awaited "downtown."

Developers of the project, scheduled for completion in 1996, declined to give a specific cost, describing it only as a "major, major undertaking" that would cost "several hundred million dollars" but less than $500 million. The 85-acre tract would form the core of the 460-acre area identified as downtown in Reston, the planned community about 18 miles west of Washington in Fairfax County.

The downtown core would ultimately hold two hotels, 10 to 12 high-rise office buildings, 300,000 square feet of retail space and 600 to 800 low-rise housing units, as well as an eight- to 10-screen movie theater, 8,000 square feet of gallery space and an open plaza with fountains and space for outdoor festivals, according to the plans made public yesterday.

Much of the core, which would be bounded on the south by the Washington and Old Dominion Regional Trail and on the east by Reston Avenue, would have to be rezoned for higher density development before construction could begin. The project also includes nine acres for housing east of Reston Avenue.

The entire 460-acre tract known as the Reston Town Center, northwest of the Dulles Access Road and Reston Avenue, and extending west as far as the proposed Springfield Bypass, would be developed with office and other uses at a lower density than the downtown core.

"We are going to have the ability to create a downtown; it is going to be very urban in flavor," said Greg Friess, executive vice president and general manager of the Reston Land Corp.

The Town Center is one of several large developments, now planned or under way around the area, designed to bring the activity and mixed purposes of urban downtowns to areas that once were suburban bedroom communities.

The Bowie New Town Center project would create a $400 million complex of residences, office buildings and stores on 275 acres near Rtes. 301 and 50. In 1984, the Prince George's County Council approved a massive rezoning of 488 acres near Laurel for the planned Konterra minicity.

Plans for an urban town center have been part of Reston's master plan since the 1960s, but developers said Fairfax County's recent growth spurt has brought enough workers and homeowners to the area to support a busy downtown.

"There have been several attempts in the past to bring this type of project forward, but Reston wasn't ready for it yet," said J. Hunter Richardson Jr. of the Reston Land Corp. As a neighbor to the growth around Dulles International Airport, "Reston is now emerging as a commercial market."

"This is really the final star in the crown for Reston," said Fairfax Supervisor Martha Pennino (D-Centreville). "Some people will say it's too much density or height," she said. "But we're talking about an urban design, a city design . . . . It will generate traffic, but people shouldn't expect to get into an urban core and travel 55 miles an hour."

The first phase of the urban core, scheduled to be built from the spring of 1987 to early 1989, would include a hotel with about 20 stories, a 16-story office building and a 10-story office building, restaurants and retail shops lining the north and south sides of the project's "Main Street," and an open plaza.

Joe Stowers, a member of the Reston Community Association's planning and zoning committee, said the project would provide services and cultural amenities not found in Reston's existing "village centers" of neighborhood shops. "They're really convenience centers," Stowers said. "You still have to drive 10 miles to buy your wife a dress for her birthday."

Reston Land Corp., the developer of Reston since 1978, is planning the project in partnership with Himmel/MKDG, a real estate development firm based in Chicago and Boston that has worked on similar mixed-use projects in those cities.

For the Town Center core to proceed, the bulk of the 85-acre tract must be rezoned from low-density residential to planned residential community, the special mixed-use designation that applies to most of Reston and permits development blending various building heights, open spaces and residential units in the same area. Public hearings before the Fairfax Planning Commission and the County Board of Supervisors are likely to take place this summer.