The Fairfax County Board of Supervisors voted yesterday to require condominium developers to foot the moving bills for tenants forced out of their converted apartments, and to provide elderly and disabled tenants with lease extensions up to three years.

The ordinance, although supported by tenants' groups, signaled a weakening in the county's power to deal with condominium developers.

Fairfax officials previously used zoning regulations as a way to secure concessions from condominium developers. A bill enacted by the Virginia General Assembly this year forbids such dickering after Thursday, and requires that county zoning powers apply only to zoning matters.

The act, introduced by Republican Del. John H. Rust Jr. of Fairfax, enables local governments only to require that developers pay relocation costs and provide the lease extensions to elderly and disabled people. The number of lease extensions is limited to no more than 20 percent of the units in a building under the new law.

The Alexandria City Council adopted an identical ordinance earlier this month, and the Arlington County Board approved a measure last night similar to Fairfax's, providing for up to $500 in relocation expenses to tenants forced from their homes. The Arlington board also enacted an ordinance providing the disabled and elderly up to three consecutive one-year leases in developments that have been converted to condominiums.

Several Fairfax supervisors said yesterday that despite the new state legislation, the county is not hamstrung in dealing with the condominium devlopers. "Certainly the leverage we have has decreased," said Providence Supervisor James M. Scott, a Democrat. "But we still have some flexibility."

"I think the counties still do have flexibility," Rust said later in an interview. "But the flexibility comes within the proper scope of the counties' power."

Scott, citing possible legal hazards, would not say which areas might cause the Fairfax board to deny an application for a condominium conversion. "I don't want to give any developer ground for a lawsuit," he said.

Fairfax has already been sued by one condominium developer it attempted to block. Owners of Arrowhead Apartments, a community of 343 homes near Tysons Corner, charged that in a lawsuit that the county has no legal right to block the project's conversion.

Mason Supervisor Thomas M. Davis III, a Republican, predicted that more lawsuits are likely. "The courts are going to have a field day construing our ability to deal with this thing."

Some supervisors also saw problems with their new ordinance. The law "only says that the leases for the elderly and disabled have to be at market value," said Annandale Supervisor Audrey Moore, a Democrat. "But are they market rentals before or after the apartment goes condo?" Moore said the rent for a condominium apartment, judged by its market value, could "go from $350 to $600 just overnight."

The Fairfax ordinance passed yesterday is effective Thursday, and applies to all condominiums or cooperatives in the county.