Fairfax County will use its new affordable housing fund for the first time next week when it pays a developer to preserve rental apartments in Centreville that could have been converted to luxury condominiums, officials said yesterday.

County officials said the Board of Supervisors on Monday probably will agree on a $2.5 million contribution to Wesley Housing Development Corp., a local nonprofit group that will take ownership of Madison Ridge and preserve 98 of the 216 apartments at below-market rates. The remaining units will be converted to condominiums, but Wesley officials said they hope to offer discounts to as many buyers as possible.

"The alternative was a conversion," Rosana Montequin, director of real estate operations for the Fairfax-based group said yesterday. "No one will be evicted. We are looking to break a record . . . and find a way to give as many residents as possible discounts to buy."

The $34.5 million purchase of Madison Ridge, a three- and four-story complex on Rydell Road with monthly rents ranging from $1,040 to $1,230, is a victory in the county's effort to preserve and build below-market housing in one of Washington's most affluent suburbs. With a molten housing market showing no signs of cooling, Fairfax leaders are concerned that the county's growing workforce -- from county government employees to its vast sector of service workers -- can't afford to live there.

At the start of the fiscal year July 1, the county dedicated $18 million to create affordable housing, augmenting a much smaller trust fund that accumulated largely from developers' contributions.

"There is a tendency to forget that there are pockets of lower-income residents here," said Supervisor Michael R. Frey (R-Sully), whose western Fairfax district includes Madison Ridge. "The county's ability to provide some funding [for Wesley's purchase] was critical."

The supervisors approved the $18 million with the budget in April as an annual stream of revenue that could help meet one of board Chairman Gerald E. Connolly's priorities. The goal is to create, through preservation or construction, 1,000 below-market apartments, condominium units and townhouses by 2007. The county is about a quarter of the way there, having financed the redevelopment of 102 apartments in Reston and other scattered projects.

"We're open for business," said Paula Sampson, director of the Fairfax County Department of Housing and Community Development, which is overseeing the "Penny for Housing" fund.

The new fund adds to a variety of federal, state and county housing programs to help first-time homebuyers and low- and moderate-income residents live in or stay in Fairfax. The county has teamed up with Wesley and other nonprofit housing groups. "But the market was softer then," Sampson said. "Now it is getting to the point where we're getting outbid, and we need to step up and help the nonprofits compete."

Madison Ridge, bounded by streams on two sides and with the remains of a historic grain mill on the site, is the kind of property the county hopes to target for preservation: a development built about 20 years ago that's still in good shape and ripe for private condominium conversion.

"The feeling is we have to preserve what we have," Sampson said. "It may not be all that affordable. But it could get a lot more expensive."

The county will contribute a total of $8.6 million to the purchase of Madison Ridge, which was being eyed by several market-rate developers before Wesley's bid, officials said. Montequin said she expects the rents to stay stable and condominium prices to hover at $225,000 for a one-bedroom unit and $290,000 for a two-bedroom unit. Those are the current market rates, "but we certainly hope to do better than that," she said.