Less than 40 miles from Washington, James H. Marshall's Fairfax County ranch house is shaded by enormous oaks in a quiet woodland glade. When he cleared the brush off his 2-acre plot and built the house 14 years ago, Marshall says, he was seeking "isolation in pristine nature."

Nowadays, Marshall is fighting hard to preserve the solitude of his Great Falls retreat as Fairfax officials consider a plan that would permit the construction of a major commuter route alongside his property.

Called Holly Knoll Drive, the 3-mile-long, two-lane road would connect two planned residential developments in neighboring Loudoun County with Rte. 7, the area's main road into Washington. And it would slice right through Marshall's wooded hideaway.

"That road would destroy the community," says Marshall, who is working with a group called Safe Transportation Options (STOP) in an effort to halt the road. "To come in with a road to serve a community that doesn't exist and cut up a community that does exist--it just doesn't make sense to me."

The dispute seems likely to come to a head Monday night when the Fairfax County Board of Supervisors considers a proposal by Washington developer Warren K. Montouri to build the $2.5 million road link on land he has purchased.

Montouri, who is planning 2,000 homes in his Cascades development in Loudoun, has left the door open for a lawsuit if his request is turned down. He argues that the land was purchased and the road planned before most of the nearby Fairfax homes were ever constructed.

Marshall is one of hundreds of local residents caught up in a squabble that started years ago, when Loudoun began a systematic program of high density residential growth along the border it shares with Fairfax. Just across the border from those planned subdivisions lies one of Fairfax's most rural areas, with horse farms, estates and secluded homes nestled along winding, wooded country roads.

Loudoun residents and officials insist that the conflict in goals between urban eastern Loudoun and rural western Fairfax arose out of an honest desire by Loudoun to maximize its development potential. It only made sense, said Loudoun County Administrator Philip A. Bolen, for Loudoun to concentrate its heaviest development in the area closest to Washington--an area that, coincidentally, is not as well-suited to farming as is the rest of the traditionally agrarian county.

Besides, said Bolen, many crucial planning decisions in the area were not made by county officials at all, but by the courts, in a series of decisions over the past decade that came down heavily in favor of the rights of developers.

Such regional conflicts, say national planners, are becoming commonplace around the country as metropolitan area populations spread out into once rural areas.

"Each locality has its own idea of how it wants to develop, and it doesn't want to give up its power," said Gregory Longhini of the American Planning Association, which represents urban and regional planners across the country. "You might think that a regional planning approach would make more sense, but political decisions aren't usually made that way."

The Fairfax supervisors see themselves faced with an unpleasant choice. Do they allow the developer to build the road with his own money on his own property, angering the Holly Knoll residents? Or, do they block the roadway plans and and force traffic onto the back-country roads through Great Falls, ultimately sticking taxpayers with the bill for improving those roads? County officials say the supervisors have the planning authority to decide whether the road should be built.

"Any right-minded person would say that neither of these courses is good," says Supervisor Nancy Falck (R-Dranesville), who now says she will support the road that will cut through her district. "We're just looking to see which is the worst case." Residents of Holly Knoll, a cluster of 250 brick colonials just south of Marshall's house, say the road would exacerbate traffic tie-ups on already overburdened Rte. 7 and cause safety problems in their quiet community. "The road shouldn't come through here at all," says Ronald Hinckley, a White House special assistant who is heading STOP. "It would split our community, separating 100 homes from ballfields and soccer facilities. We have lots of kids in the community, and it wouldn't be safe for them to ride their bikes."

A better solution, they say, would be to build a $6 million bridge over Sugarland Run to link the two developments with the planned "Algonkian Parkway," a route that would carry traffic westward and south to the Dulles Toll Road for the commute into Washington. But so far the state has declined to finance the bridge.

It is estimated that the 2,500 new homes slated for the Great Falls Forest and Cascades developments in Loudoun would generate about 20,000 new car trips a day in the area--more than enough to add significantly to Rte. 7 tie-ups that already create mile-long traffic jams near Holly Knoll every day.

Fairfax officials say, however, that traffic generated by those two developments probably will be minor compared to what is in store from another 7,500 homes in Loudoun that also have won approval for construction in the vicinity. A huge shopping and office development is also being considered in eastern Loudoun.

"It's called dumping your traffic on your neighbors," grouses Falck.