A group of Potomac residents, hoping to block construction of 850 homes near exclusive River Road, are trying to reverse a controversial Montgomery County law that has preserved hundreds of acres of agricultural land while allowing higher-density housing in more congested areas of the county.

"We are challenging 'checkbook' zoning," declared Sarah Greening, president of the 500-member civic association in Potomac that appealed a county planning board decision allowing construction of a $7 million golf course and a companion 850-unit housing development on the old 1,000-acre Avenel Farm near the Potomac River.

At the core of the appeal, recently filed in Montgomery County Circuit Court, is an attempt to kill so-called transfer of development rights -- TDRs in planner jargon -- a relatively new planning concept that has been used to preserve historic sites such as Pennsylvania Station in New York and the Pine Barrens forest in New Jersey, among others.

Montgomery's widespread use of such transfers has attracted worldwide attention in the three years since its TDR program began.

During that time, the county set aside 1,000 acres of rich upcounty farmland and simultaneously spurred denser development along such busy commercial corridors as Rte. 29. As many as 10,000 more acres of farmland may soon be added to the program, officials said yesterday.

The county law enables a developer to add extra buildings on a parcel of land after agreeing to buy up the rights to a corresponding amount of agricultural land, thus saving it from intense development.

Greening said developers of the Avenel project, who utilized the TDR law to double their planned housing, will put "an extreme burden on our land" in Potomac. TDRs circumvent standard zoning laws, are unconstitutional, strain local services and lead to more congested roads, Greening said.

But Norman L. Christeller, chairman of the planning board that approved the Avenel subdivision two months ago, said TDRs were crucial to the preservation of vast stretches of farmland. "We're going to be in a hell of a mess if they do succeed," Christeller said, referring to the court action by Greening's West Montgomery County Citizens Association. "The whole point is that TDRs give us the assurance of perpetual easements against development on agricultural land."

Under Montgomery's TDR program, "you're buying your zoning, all right, but you're buying it from a landowner who has given up his right to develop that property," said Christeller, adding that a successful court challenge could bring a halt to several major housing subdivisions in the county.

Christeller also noted the failure of a 1981 suit against the TDR concept and said he expects the suit by the Potomac group to suffer a similar fate.

Stephen Z. Kaufman, attorney for the Avenel Farm developers, said the Potomac civic group was "trying to retry" the 1981 case. He said the developers still "intend to go forward" with their plan to build 850 homes on the site.

A county circuit court judge is expected to hear the case within six months.