A Fauquier County judge yesterday upheld a controversial decision by county officials allowing developers to carve a large subdivision out of the rolling farmland southwest of metropolitan Washington.

But Circuit Court Judge Rayner V. Snead, who heard the lawsuit brought by a coalition of farmers and activists, rejected a county argument that could have had statewide land-use implications.

The ruling approved an April 1978 vote by the Fauquier Board of Supervisors to allow the building of Partridge Run Estates, 22 parcels of 10 to 15 acres each to be divided from 373 acres of existing fields and pastures.

Snead said the suit by five local farmers and the Mid-Fauzuier Association, a citizens' group, "did not provide sufficient evidence that the board acted improperly" in reaching its decision a year ago.

The judge rejected a broader argument by the county, however, that it could approve the proposed development without being limited by its own statemandated comprehensive plan for future growth.

The Fauquier plan recommends, but does not require, that residential building in the county be confined to so-called "service areas" that protect the county's remaining agricultuarl areas. Snead said the citizens' group failed to proved the supervisors acted without regard for the provisions of the plan.

Both sides in the lawsuit yesterday expressed pleasure at the outcome.

"We did the right thing here," said Supervisor Robert Gillman, a supporter of development in the county. "Partridge Run is not a subdivision in reality, not what they think of as subdivisions in Fairfax or Arlington. This is not urbanization. What we've done is cut 300 acres into 22 small farms."

Allen Olson, the lawyer for the citizens, hailed Snead's rejection of the county's argument as an "important victory" that would force the supervisiors to protect agricultural lands in future cases.

Olson said yesterday's decision "invited an appeal," but said the group would make up its mind later in the week.