The Fairfax County Board of Supervisors agreed yesterday to allow builders to make certain construction site inspections currently performed by county employes, but only after insisting the county retain the final say on all projects.

The proposal, backed by the influential Northern Virginia Builders Association, was approved on an 8-to-0 vote after nearly two hours of debate and a handful of stipulations.

"The county will have the final approval," said Supervisor James M. Scott, a Democrat who expressed satisfaction with the new system after earlier voicing reservations.

The proposal covers inspections that measure the degree to which public improvements such as sewers, gutters, sidewalks and storm drains are complete. As builders complete the improvements, they are allowed to gain release from the bonds they are required to post as a promise that the public improvements will be made.

County officials said the new procedure is a response to a Virginia law, effective July 1, that loosens the bonding requirements localities may impose on builders.

Under current law, developers may ask for reductions in their subdivision bonds only once during a construction project. If county inspectors find that a substantial portion of the sidewalks, gutters, sewers and private roads are complete, a reduction is granted.

Under the new law, builders may ask for three reductions a year for their projects.

County officials, who now check each project before signing off on bond reductions, said the new law could cause a sharp increase in the requests for bond reductions. They called for a new system in which builders would hire their own inspectors to perform the necessary checks.

County officials said they would still review the quality of the builders' inspections through spot checks.

"I don't see dramatic changes," said County Executive J. Hamilton Lambert. "You spot-check everything. You have inspections all through the project."

County workers will still "evaluate the quality" of public improvements said Director of Environmental Management Claude G. Cooper. "[But] I do not have the resources to measure the length of sewer pipes on every project or verify all the calculations."

Cooper said the county now gets about 150 bond reduction requests a year, but that he expects that figure to jump to 4,000 to 5,000 after July 1.

Most of the supervisors expressed their unease with the new system, and adopted a number of provisions designed to make sure it works, and give the county a way out if it appears unsatisfactory.

The county board adopted a proposal by Supervisor Thomas M. Davis III that the new system be reviewed in six months. The board also voted to look into an alternative system, under which the county would hire an engineering firm to do the inspections.

"If we find that the trial is not successful," said Scott, "we don't want to be in the position of not having an alternative to go to."

In yet another stipulation suggested by Davis, the board voted to require that private engineers paid by builders to perform inspections swear an affidavit that their work is accurate.

Fairfax is undergoing a construction boom that has put all county inspectors under intense pressure. Most other Washington area jurisdictions do not permit builders to finance inspections of their own projects.

Some supervisors have said they fear the new system may give rise to cozy relationships between builders and the engineers they hire to do the required inspections.

Board vice chairman Martha V. Pennino, one of the supervisors who had been critical of the proposal, was the only supervisor who did not vote for the proposal. She was absent from yesterday's board meeting.