An obscure board in the Fairfax County government has in effect overturned a decision by the Board of Supervisors that allowed expansion of a gas station in an area of McLean that county officials were trying to maintain as residential.

The Board of Zoning Appeals, which usually devotes its weekly meetings to such weighty questions as whether a homeowner can build a fence outside a required distance on his property, ruled on Wednesday that the small Langley Exxon Servicenter at Dolley Madison Boulevard (Rte. 123) and Kirby Road can expand onto land the county maintains is zoned for homes.

The McLean Citizens Association and some county officials fear expansion could lead to a commercial strip along Rte. 123 outside of McLean's central business district, which is two miles from the service station.

"It's clearly illegal," said Lilla Richards, first vice president of the McLean Citizens Association. "It's a usurpation of the powers of the Board of Supervisors."

Though the appeals board is part of the county government, it is completely independent of the supervisors. For years, the supervisors have tried - in vain - to gain control of the board, whose five members are appointed by the chief judge of the Circuit Court.

Critics of the present arrangement say it is a vestige of the days when the county government was run largely from the courthouse.

The supervisors could take the appeals board to court, which would amount to one part of the county government suing another part.

Also caught in the middle is County zoning administrator Gilbert R. Knowlton, who, as he says, "works for the Board of Supervisors but with the Board of Zoning Appeals." Knowlton argued against the service station expansion at the board's meeting on Wednesday.

The board spends much of its time hearing appeals to decisions by Knowlton's office on what homeowners and businesses can and cannot do in improving or altering their property. It also rules on requests for special uses of land that do not conform to existing zoning regulations.

How the Langley Servicenter issue got before the board, after the supervisors rejected a commercial rezoning last December, is part of a complicated story that begins in 1936.

The original station was built that year, before Fairfax had a zoning ordinance setting up residential and commercial areas.

In 1957, the supervisors, at the request of the Wagner family, which owned the land where the station was located and a number of surrounding acres, rezoned 4,068 square feet at the corner of Rte. 123 and Kirby Road to general business district.

The rezoning permitted the station to add some pumps to serve motorists from Kirby Road. More significant than that, it provided a basis for the landowner to claim later - before the Board of Zoning Appeals - that the commercial area was not limited to 4,068 square feet, but was actually almost 10 times as big, or 40,000 square feet.

But before the case came before the appeals board, attorney William H. Hansbarger, representing the estate of Mabel V. Wagner, sought a commercial rezoning for the 40,000 square feet before the supervisors last December. The Board of Supervisors rejected the application.

Hansbarger's next stop ordinarily would have been to appeal the supervisors' decision to Circuit Court. But instead the attorney chose a tactic that would bring his case before the Board of Zoning Appeals.

He argued before zoning administrator Knowlton that the service station's business area was 40,000 square feet, based on the "rules for interpretation" in the county zoning ordinance that says that "the depth of (a commercial) strip, unless otherwise shown on the map, shall be deemed to be 200 feet, measured at right angles, from the sideline of the street to which such strip is parallel and adjacent."

Knowlton, on the other hand, said the rule cited by Hansbarger was preceded by another one that says the size of a zoning district is to be determined from a scale map when the dimensions of the districts are not listed.

Knowlton said the county's scale map indicated the size of the commercial parcel is about 4,000 square feet, or roughly the amount of land zoned for business in 1957.

He also said the earlier rule - and not the one cited by Hansbarger - would apply because scale maps have been used since 1967.

F. Tyler Swetnam, who was part of the majority in the 3-to-2 decision to overrule zoning administrator Knowlton, said in an interview, "You couldn't build a high-rise birdhouse on 4,000 square feet. An injustice was done, and it was up to us to correct it. If we're wrong, the court will overrule us."

Hansbarger added a new element to the controversy yersterday when he said he would propose that once the station's expansion is guaranteed, all the station's commercially zoned land - be it 4,068 or 40,000 square feet - be rexond residential. Such a move, he said, should lay to rest any fears that a larger station might lead to a commercial strip along Rte. 123.