Fairfax County supervisors have grown skeptical of a proposal to legalize what many suburban homeowners long ago made commonplace: the rental of basement apartments and other self-contained rooms in single-family homes.

County officials had promoted the "granny flats" concept as a means of providing affordable housing for the elderly and handicapped while offering some income to some older homeowners who might otherwise be unable to afford their property taxes. At a public hearing on Nov. 1, the plan was endorsed by everyone from the county social services board to the Northern Virginia Board of Realtors.

Since the hearing, however, less well-organized opposition has developed from homeowners who fear the change in zoning laws could alter their neighborhoods and decrease the values of their homes. The County Board of Supervisors, scheduled to consider the proposal Monday, now seems likely to reject it or send it back for further study.

"There's a real outcry against it now," said Annandale Supervisor Audrey Moore, who said she is looking for a compromise that would allow a small-scale trial. "People are worried it's going to downgrade the neighborhoods."

The current Fairfax proposal would allow homeowners to rent "accessory units" within their homes or garages as long as the outside appearance of their homes did not change. The occupants of either the main dwelling or the new unit would have to be handicapped or at least 62 years old, and the county would have the option of revoking permission for the separate apartment every two years.

"We still have a long way to go to provide adequate housing for the elderly and the handicapped," said Providence District Supervisor James M. Scott, "and this is one method that's been used around the country. I haven't heard any reason not to give it a try."

The Fairfax law would be the first of its kind in this area, according to a survey by the county staff, but it would respond to changing demographics common to most area suburbs. The average household size in the region declined from 3.09 people to 2.67 between 1970 and 1980. Meanwhile, the median value of homes increased from $53,000 in Fairfax in 1973 to $104,100 this year.

Patrick Hare, a city planner who testified at the board's Nov. 1 hearing, said more than 2 million accessory units have been set up in the country, many of them illegally. "The significance of that data is that no one has noticed except the Bureau of the Census," Hare said, arguing that granny flats have had only positive impacts on neighborhoods.

But the one person who testified against the proposal at the public hearing said the county would be unable to enforce its restrictions. "Residents of Orange Hunt, like in many other communities, purchased their homes with the idea it would be a single-family neighborhood, and they put a lot of money behind this," said Casey Goutos of the Orange Hunt Estates Civic Association.

Goutos described her neighborhood in western Fairfax as middle to upper-middle class and she said her association supports the intent of the change. "But when you threaten someone's home, that's almost as tricky as threatening someone's life," she said. "This is the expense of a lifetime."