Four months after Fairfax County won a landmark court decision that limits development in the rural southwest quarter of the county, citizens groups are attacking a new proposal that would allow construction of day-care centers, nursing homes and private schools there.

The groups say the proposal -- an amendment to a zoning law -- would seriously compromise the county's victory in the court case in which a judge upheld the right of the County Board of Supervisors to restrict building to one house per five acres of land in the 38,500-acre Occoquan River watershed.

The county spent more than $500,000 defending the court challenge to its authority filed by a group of developers. Its victory was hailed as affording protection to the environment in the last large undeveloped area of the county.

"We are shocked by this proposal," said Sally Ormsby, president of the Fairfax County Federation of Citizens Associations. "It seriously undermines the court decision."

"This is a conservation district which they've set up," said Sue Anderson, president of the League of Women Voters of the Fairfax Area, "and once you allow this type of development, it destroys the conservation district."

But supervisors said the zoning amendment, which is backed by county planners, is designed to allow existing homes and businesses flexibility in minor expansions. They said the proposal is likely to be watered down before it goes to the supervisors June 3 for a decision.

Elaine McConnell, a Republican supervisor whose district includes much of the affected land, said she would support a compromise. "We will come up with something that's going to satisfy everyone," she said. McConnell is a leading advocate of the proposal.

"I'm going to make sure we don't lose anything we gained in the Occoquan case," said Supervisor Thomas M. Davis III, a Republican.

The proposed zoning amendment would, under some circumstances, permit broader use of the land than currently allowed. For example, the supervisors could grant special exceptions clearing the way for colleges, nursery schools or housing for the elderly. The county Board of Zoning Appeals would be empowered to grant special permits for smaller-scale developments.

The citizens groups contend that those projects are inconsistent with the intent of the law passed by the county in 1982 limiting development in the Occoquan area. The land drains into the Occoquan Reservoir, which is the source of drinking water for about 600,000 Northern Virginia residents.

Board Chairman John F. Herrity, a Republican, said the county will be vulnerable to lawsuits from owners of existing enterprises unless it allows them the flexibility to expand.

The board appointed McConnell and Supervisor Audrey Moore, a Democrat, to discuss the proposed zoning amendment with the citizens groups. They are scheduled to meet May 23.