The Fairfax County Board of Supervisors approved a giant office park yesterday that could cost up to $2 billion and rival the Tysons Corner area in office space despite warnings from area residents that the project will bring severe traffic jams to the already congested Rte. 28 corridor.

The 1,048-acre Westfields project, which could attract about 30,000 workers daily by the year 2000, is one of the largest commercial projects proposed in Fairfax County. It includes a hotel and conference center, offices and warehouses. Roads for the project already are being built, and construction is scheduled to begin next year.

The project is seen by county planners as a catalyst for development along the already booming Rte. 28 corridor, which skirts Dulles International Airport to the east.

The area near Dulles, which straddles the western edge of Fairfax, is expected to be one of the fastest-growing areas in the metropolitan area during the next decade. Some fear it also may produce more traffic than roads in the area can handle.

County Board Vice Chairman Martha V. Pennino (D-Centreville), who supported the Westfields project, said resignedly, "Whatever you do is going to create a traffic nightmare. We're not like Kentucky or Tennessee. We're next door to the nation's capital. We're a city. We're urban."

"I just don't see how the area's going to cope" with the traffic, said Supervisor Audrey Moore (D-Annandale), the lone dissenter in the board's 5-to-1 vote to allow the rezoning necessary for Westfields. "The magnitude of this project is so incredible versus the ability of the county to provide roads. You're not going to get from here to there."

But officials from Westfields, which is being developed by Henry A. Long Co., stressed that they are building $30 million of new roads as part of their project. And county officials said that Long and other developers will build whatever roads are necessary to ensure the success of their projects.

Much of the land south of Dulles where Westfields is to be located was zoned for industrial and office uses in 1982, at the same time the board restricted development on hundreds of acres in the county's southwest, an area called the Occoquan Basin. That land, a watershed area that drains into the Occoquan Reservoir and provides 600,000 Northern Virginians with water, can be used today only for homes on large parcels.

In part, the Dulles area is a focus for development today because the Occoquan zoning decision left so little acreage available elsewhere in the county for major projects.

"This project is significant," said Westfield president Francis C. Steinbauer. "It will be larger than Tysons, though less dense."

One major concern about Westfields is that it will add to the already heavy traffic on Rte. 28, also known as Sully Road, a key north-south artery that runs through Fairfax, Loudoun and Prince William counties. Westfields straddles Rte. 28.

Officials have stressed that development along Rte. 28 will be choked off unless the road is widened, but until recently there were no definite ideas about how such a major plan would be financed.

Last month, some county board members proposed a special tax district to pay for widening Rte. 28 to four or six lanes. The tax district, board members said, would force developers to pay most of the cost of widening the mostly two-lane country road.

County planners and transportation officials stress that developers along the Rte. 28 corridor will have little choice but to pay for widening the road themselves. If they don't, they said, their projects will be inaccessible because of the traffic.

The county board postponed a vote on the Westfields project Oct. 28 when it failed to reach an agreement with Long Co. representatives to provide road and other improvements.

Last week, a vote on the project was again delayed because county board members complained that the developer had not provided full disclosure, as required by law, of all the members of the partnerships in the Westfields venture.

In other business, County Board Chairman John F. Herrity blasted a plan by state highway officials to build a four-mile bypass around Purcellville in Loudoun County at a cost of $6 million to $10 million.

He said that to build a costly bypass around the small town (pop. 1,600) when more populous areas in Fairfax were badly in need of road improvements is "a disgrace."