The Fairfax County Board of Supervisors yesterday vote unanimously to grant a substantial tax break to 623 acres of woodlands and riding stables owned by a major developer, who promised in return not to develop his land.

Edwin Lynch, the 69-year-old landowner who successfully applied for the county's second agricultural district, said the tax break will mean the difference between red ink and profit for his Meadowood farm, which boards 45 horses for local equestrians and grows hay and other fodder.

Lynch and his family, much of whose land has been developed into subdivisions and offices in Springfield and Annandale since World War II, still own $26 million of property in Fairfax, according to county records. Lynch said yesterday he plans to build his retirement house on a bluff overlooking the Potomac on the horse farm south of Mount Vernon.

The board approval will lower the farm's property tax from $20,000 a year to less than $3,000.

Supervisor Thomas M. Davis III questioned the need for the tax break, saying the land is not served by sewers that would make development likely. "He's one of the biggest developers in the area, and he's getting a tax break," Davis said.

Other supervisors said the land is surrounded by parks and nature preserves and its preservation should be encouraged. Supervisor Nancy Falck said county taxpayers are protected because Lynch would have to pay five years of back taxes if he develops the land.