The Fairfax County Board of Supervisors took two separate steps yesterday that are designed to place more controls on development in the county, the most rapidly growing Jurisdiction in the Washington area.

The board rejected pleas from builders that it suspend its new home disclosure law and, in a second action, directed builders to undertake strict pollution controls in any projects near the Occoquan Reservoir. The reservoir is the source of drinking water for more than 600,000 Northern Virginians.

Moments after the supervisors turned aside requests that the county suspend its disclosure law, an official of the Northern Virginia Builders Association said his group would file a lawsuit against the county within a few weeks to have the law overturned.

Cecil M. Boyer Jr., the builders' spokesman, told the board the homebuilding industry should be allowed to develop a voluntary disclosure law before the county proposal, the first of its type in Virginia, is implemented. "The Board of Supervisors . . . is having a political heyday on consumer issues which demonstrably can be self-regulated," Boyer said.

Earlier the supervisors unanimously approved four minor amendments to the disclosure law, which requires builders to inform new home buyers in writing where they can go to make complaints, seek legal advice and obtain data on utility costs and other considerations.

The law, which took effect a week ago, was enacted last March in response to rising consumer complaints about construction defects in new homes.

Boyer told the board he thought the law was discriminatory because it applied only to new home sales. These constitute only one-sixth of annual home purchases in Fairfax, he said.

The supervisors, all of whose seats are up for election this fall, did not act on Boyer's request that they suspend the ordinance for at least six months.

Boyer also cited an opinion by Virginia Attorney General J. Marshall Coleman, who said the supervisors did not have authority under state law to enact the ordinance.

The supervisors complained yesterday that the builders only came up with their voluntary program when faced with the board's ordinance. "My feeling is it's just too little and too late," said Marie B. Travesky (R) whose Springfield district is one of the county's fastest-growing areas.

Supervisor Alan Magazine (D-Mason) said the builders had not attempted to deal with consumer complaints until the county stepped in. "It means to me the industry is not particularly concerned with solving its problems," Magazine said, "What it's concerned about is keeping the county off its back."

Of the five supervisors voting on the pollution controls, only Audrey Moore (D-Annandale) voted no. She said she wanted the stiffer controls involving treatment of runoff waters and, more importantly, stiffer limits on how many houses developers can build in the Occoquan area.

The pollution controls do not place limits on the number of houses in the Occoquan watershed, which includes the fast-developing area east and south of Dulles International Airport. Last year the supervisors, rejecting proposals in their own master plan, decided not to put stricter limits on how many houses can be built.

Pollution from development there "could be dramatically reduced" under the new policy, the supervisors were told by Planning Director Theodore J. Wessel.

Without the controls, Wessel said, the Fairfax County Water Authority, which depends on the reservoir for supplies, might have to build carbon filters that would cost $28 million. Worse still, a recently opened $82 million regional sewage treatment plant upstream from the reservoir "would be compromised," he said.

The new county policy will strenghten existing pollution controls - generally by requiring builders to construct more elaborate detention ponds to trap more pollutants.

Wessel said the additional controls would add only about 15 percent to the cost of present controls. He said the county staff had rejected more sophisticated controls that would have required chemical treatment of runoff water because they were too expensive.