In Fairfax County, where only the affluent ordinarily can afford to buy a home, the County Board of Supervisors took a step yesterday toward preserving and improving a mobile home development where the median family income is only $10,500 a year.

The supervisors told the county Department of Housing and Community Development to draft plans to use federal funds to purchase the Woodley-Nightingale Mobile Home Park on Rte. 1 in the Groveton area, then turn it into a cooperative run by residents.

The supervisors' unanimous action does not commit them to carrying out the housing department's plan, but was "a vote in principle" for the concept of a cooperative type of trailer park, according to Gerald W. Hopkins, director of the county's quasi-independent redevelopment and housing authority.

If the plan outlined yesterday is carried out, it will be the county's first redevelopment project.

The 44-acre Woodley-Nightingale development is one of 11 trailer parks in the county. About 1,300 persons live there in 500 mobile homes. About 80 per cent of the families own their mobile homes.

The county's interest in improving conditions at Weedley-Nightingale stems from residents requests for help when they faced the possibility of having to move about three years ago because the park site was up for a zoning change to commercial use.

"We need another shopping center on Rte. 1 like we need a hole in the head," said Emma Daniels, treasurer of the park's civic association. "Where would all the families go?"

Both county officials and residents of the trailer park complained yesterday about poor electrical wiring; rugged, narrow roads; lack of packing spaces, and crowding. The nearest fire hydrant is on Rte. 1 about a half mile from some parts of the park.

County officials told supervisors that the present owner of the park, Kirby and Wren Associates, indicated they could not make the necessary improvements in the park because of the large investment required.

Under the plan outlined yesterday, the park would be purchased by the county with a federal community development block grant.

Park residents would then form a cooperative and apply for a Federal Housing Administration loan for improvements.

The cooperative would repay the loan and cover operating costs for services such as playgrounds and a community center (which do not now exist) out of revenue generated by a monthly fee of about $95 from each trailer "pad." A pad is the lot on which a trailer sits and includes electrical, water and sewage connections.

Those paying the monthly charge would participate in the cooperative much like a shareholder owns stock in a large company. If a person decided to move, he would be reimbursed for his investment according to a formula written into the cooperative agreement.

Residents queried yesterday said they would like to see the county implement the cooperative plan, which County Executive Leonard Whorton yesterday called a "major undertaking."

"I think that's a grand idea-everyone owning their own lot and being responsible for keeping it up," said Olivia F. Woods. "I don't think a person would mind paying a little bit more if they could have a larger lot."

Daniels said her civic association has endorsed the cooperative plan. "I think it would be a great incentive. If you own something you have more pride in it," she said. Daniels has lived at Woodley-Nightingale for 13 years. "I prefer a mobile home. I cannot afford to buy a home in this area. The prices are outrageous.Why should I pay $300 to $400 a month for an apartment that still belongs to someone else? This trailer belongs to me."

Many residents of the trailer park, which was built in the 1940s, have lived there for 15 or 20 years. Daniels said. "So many people think it's a transitory life, but it's really permanent. I own land in West Virginia and when I go back I'll probably put a trailer on it."

The median price of a home in the county is about $60,000, and many county employees cannot afford to live there. There are 400 housing units for low- and moderate-income families in the county, and the housing authority has funds to subsidize the rent of 261 families. The county is building a cooperative development of townhouses in Reston for moderate-income families.