The Fairfax County Board of Supervisors voted Monday night to spend $2 million on a plan, long opposed by environmentalists, to extend a private developer's sewer lines to the Lincoln-Lewis-Vannoy community, an isolated area in southwest Fairfax County where most residents still use outhouses and have no running water.

The board approved the project 8 to 1 under pressure from the State Water Control Board, which ordered the county to scrap its $4.3 million plan for a sewage collection and treatment site for the area. The county has already spent $833,000 on the original project--money that Fairfax Board Chairman John F. Herrity, who voted against the plan, said is being "thrown away." Included in that cost are 70 acres of land the county bought for a treatment site it will not use under the new plan.

The proposed sewer project could mean the end of more than 30 years of government delays that have left the 169-home community without indoor plumbing and bathrooms. Health officials refused to allow most residents to install private septic tanks because the land is too rocky to properly absorb sewage runoff.

County officials said it will be at least two years before residents see the improvements. But some developers said it may be even longer unless the county agrees to contribute more money to the cost of extending the line.

The new proposal would cost about half the amount of the original project and save the county money, argued Glen G. Ehrich, Fairfax public works director. But only part of the costs of providing the additional pipeline, which includes a pumping station, is covered by the proposed $2 million budget for the project. Ehrich said county officials would try to negotiate a settlement with the developers, asking them to pay a share of the costs of extending the line.

State water board officials ordered the original project scuttled when it learned private developers were constructing a sewer line across the ridge that separates the Lincoln-Lewis-Vannoy community from the sewer line. Although county planners privately have been urging the move, environmentalists warned that tying one community into the sewer line could open the ecologically fragile Occoquan River Basin to rapid development.

Supervisor Marie B. Travesky, whose district includes the community, said a separate decison by the board Monday to limit growth in the Occoquan watershed would protect the area from future development involving the sewer lines.

Private developers building the sewer line in the nearby Little Rocky Run area said they will not begin construction on the line before late next fall and have no plans to extend the line far enough to serve Lincoln-Lewis-Vannoy.