The Fairfax County Board of Supervisors last night approved, by a 5 to 3 vote, a compromise plan that will permit construction of 50 cooperatively owned apartments for low- and middle-income people in the Wolftrap neighborhood between Tysons Corner and Vienna.

Both sides won a major concession. The county housing authority gets 50 units instead of the 30 it originally sought. The Wolftrap Citizens Association got the authority to scrap plans to build subsidized units that would have been rented instead of bought. The association, which represents a racially mixed and moderate income neighborhood, said rental units would lower property values.

Voting for the compromise were Supervisors James M. Scott (D-Providence), whose district includes the area and who was instrumental in getting both sides to give ground; Martha V. Pennino (D-Centreville), Audrey Moore (D-Annandale), Alan H. Magazine (D-Mason) and Warren I. Cikins (D-Mount Vernon).

Opposing it were Board Chairman John F. Herrity (R), Marie B. Travesky (R-Springfield) and John P. Shacochis (R-Dranesville). Herrity complained there wasn't an ironclad guarantee that the 50 units would be sold rather than rented, and Shacochis said the project would be "a designed slum."

Absent was Supervisor Joseph Alexander (D-Lee).

The project is unusual because it is part of a package that will include, on an adjacent parcel, 26 privately built houses in the $80,000 range -- evidence, the housing authority says, that subsidized housing does not scare away more expensive development.