A massive expansion of Rte. 28 near Dulles International Airport, a project that supporters originally boasted could be done entirely at the expense of private developers, would involve substantial amounts of taxpayer money under a state plan endorsed yesterday by the Fairfax County Board of Supervisors.

In a unanimous vote, the supervisors gave their approval to a tentative proposal negotiated between Virginia Department of Transportation officials and developers in the booming Dulles corridor that would require taxpayers to pay about 20 percent of the almost $110 million cost of the first phase of widening Rte. 28.

Landowners along the route, who would profit most from the widening, would finance the rest of the project under a special property tax surcharge authorized last winter by the Virginia General Assembly.

The public's share of the improvements would come from a pool of state funds earmarked for use on primary road projects throughout Northern Virginia.

Leading supporters, including Board Chairman John F. Herrity, originally had said the appeal of the special taxing district was that it would allow the expansion of badly congested Rte. 28 to get under way quickly, without monopolizing Northern Virginia's annual road allocation.

Even though the expansion would now require dipping into that allocation, Fairfax officials emphasized yesterday that the road widening would take place far sooner and with far less public money than if the tax on developers had not been approved.

State and county officials said they now hope that final plans for the Rte. 28 expansion can be approved in time for a groundbreaking next year. The first phase of the project would take the road from two lanes to a six-lane limited-access expressway with four major interchanges. A second phase would cost about $179 million and expand the road to eight lanes and increase the number of interchanges to 14.

Money for the project would be raised through the sale of bonds. By the time the project is finished, the bonds would be repaid with about 80 percent developers' money and 20 percent state money.

However, during the early years of the project, the public's proportion of the debt would be greater. The landowners' proportion would rise in later years as new developments increase the tax value of the property.

In other action yesterday the supervisors:Approved on a 6-to-1 vote rezoning for a 93-acre residential development by the Sequoia Development Corp. on Rte. 123 north of Lorton prison. Supervisor Audrey Moore (D-Annandale) was the lone dissenter; Supervisors Joseph Alexander (D-Lee) and Katherine K. Hanley (D-Providence) were absent.Unanimously approved a motion endorsing expanded powers for the county to draft financial disclosure laws for county officials acting on development proposals. The proposed new authority would require General Assembly approval.