The Northern Virginia Builders Asociation and a number of Fairfax Conty's most influenital zoning attornerys, already the victors in a recent series of major land use cases, have accused the county government of extracting concessions from them that are supposed to be voluntary.

"It's out and out blackmail," said one signatory to a letter of complaint the group delivered to the Fiarfax Board of Supervisors last night. He asked not to be identified.

The letter was signed by Cecil M. Boyer Jr., president of the builders' association, and 10 zoning attorneys, including John T. Hazel Jr., Marc E. Bettius, E. A. Pritchard and William B. Lawson.

"This is not a declaration of war," said Bettius, who with some of the signatories has taken the county to court in the past and won important decisions favoring developers. "We desperately seek the board's attention," he said.

What Bettius and the rest of the group have rebelled against is a system under which developers, in exchange for zoning designations they seek, agreed to donate land for schools or parks or make other land use concessions. In legalese, such concessions are called "proffers."

The proffer system, which is permitted under Virginia law, is supposed to be voluntary.But one of the zoning attorneys who signed the letter said last night, "We're being coerced . . . we're getting requirements (to make proffers) that have no basis in law."

However, Fairfax Board Chairman John F. Herrity said, "The proffer system is absolutely fair and in the public interest, and it will be continued

As the proffer system has operated in the county for about the last four years, the more concessions a developer offers, the greater are his chances of being awarded the highest zoning density possible under existing ordinances. County legal officials maintained last night that such concessins by developers are always made voluntarily "for the common good" and that the county has never demanded them.

In criticizing proffers, one zoning attorney cited the county's recent practive of asking developers in some areas to construct storm drainage systems to reduce pollution of waterways, particularly the Occoquan Reservoir, the source of drinking water for more than 600,000 Northern Virginians. He said that there was no evidence that the drainage controls the county requests effectively reduce pollution.

The attorney said that such measures are wrong not only because they are imposed selectively. "The strong resist and suffer diminished land use potential," the letter to the supervisors said. "The weak submit and pay the price . . ."

With their power to control land use sharply reduced by Virgina courts in recent years, the supervisors have used the proffer system to win what one county official calls "development that is done the right way."

In one of their actions last night, the supervisors refused to grant a requested rezoning when the applicant, the Norvail Development Corp., refused to donate a 50-foot-wide buffer along the property's edge to help forestall siltation of a small creek that feeds into Burke's Spring Branch. Instead, the supervisors granted the developer a lower density than it requested.