A delicate chain of circumstances that has formed over the past few months soon will bring Fairfax County the first rental apartments built in the past eight years without federal housing subsidies.

Oxford Development Corp., a Landover firm that has developed and manages more than 30,000 rental units nationwide, is planning to break ground in the next few weeks on a 640-unit rental complex in Chantilly and is completing site plans for a 460-unit rental complex in Reston.

"We are very excited about these projects," said Lynne Hansen, a development associate with Oxford. "Chantilly will be our first venture into Fairfax County and the first market-rate development in the county in years."

The factor that was most influential in bringing Oxford to Fairfax County--where the apartment vacancy rate is 2.6 percent--was the drop in interest rates over the last year. But Hansen said another influence was the growth of a market in western Fairfax for apartments at rents between $400 and $500.

"It really is an area of tremendous growth," said Hansen. "With AT&T moving out there and with the other high-tech firms in Reston, and IBM right down the road in Manassas, we felt we couldn't lose."

Hansen said the Chantilly and Reston projects will be similar, offering "top-of-the-line" one-bedroom, one-bedroom-plus-den and two-bedroom apartments, and that Oxford hopes to attract singles and couples without children. The two developments will have fireplaces in most units, washers and dryers in every unit, pools, saunas, racquetball and tennis courts and clubhouses.

"It's not a development oriented towards small children, though, of course, we will not exclude them," Hansen said.

For Fairfax County--where the word "family" is part of the county slogan--an apartment complex for people without children is a departure. But such a venture may be the only way to make money without housing subsidies in this wealthy county.

The Chantilly project will be financed with tax-exempt bonds issued by the Fairfax County Housing and Redevelopment Authority, the first such issue for a residential rental project in years, said Deidre Coyne, the housing authority's spokesman. The bonds will allow Oxford to obtain financing a few points below market rates and, in exchange, Oxford has promised to rent 20 percent of the units to low- and moderate-income people receiving rent subsidies.

Coyne said that still adds up to only partial help from the government and that there are no plans to provide tax-exempt-bond financing for the Reston project.

In fact, Reston residents, who last fall protested the rezoning that would allow the higher density for the apartment complex, probably would try to block such a financing arrangement.

"Reston has its share of low-income housing," said Fairfax County Supervisor Martha V. Pennino, Reston's representative on the county board. Financing the project with housing authority bonds "was not part of the rezoning request, and I would strongly resist such a proposal."

"At this point, we plan to finance the Reston project without any help, and we have several parties very interested in it," said Hansen.

Pennino said she might consider supporting an arrangement where the apartment owners would agree to keep units as rentals--rather than allowing them to convert to condominiums--in exchange for tax-exempt financing.