The Arlington County Board on Saturday will consider loaning $7.8 million to a local nonprofit organization to build an apartment house primarily dedicated to below-market-rate units for families that can’t afford the county’s expensive rents.

Arlington Partnership for Affordable Housing wants to raze the 27 Carlyn Springs Apartments and replace them with a five-story, 104-unit complex at North Carlin Springs Road and North Thomas Street. Ninety-eight of the new apartments would be affordable to people who make 60 percent or less of the area median income, which for a family of four is about $107,000. Current tenants would get rental assistance for the time they have to relocate during demolition and reconstruction.

The building would also house an underground garage ground-floor office space for the nonprofit headquarters.

Nina Janopaul, APAH’s chief executive, said the project has been planned for several years. It’s a half-mile from the Ballston Metro station and the land itself has been appraised at $10.5 million.

Arlington County, which does not have public housing, provides a pool of money to be loaned to nonprofits which build and operate affordable housing. Housing costs have risen dramatically in the area in the past 10 to 15 years, and the need for lower-than-market-rate apartments has risen as well, county figures show.

The county money would go toward construction. APAH plans to seek federal tax credits through the state’s housing development authority, and use a private first mortgage, deferred developer fees and a note from the seller to fully finance the project.

Arlington would charge a 2 percent interest rate for up to 30 years. The fund that the county sets aside for financing affordable housing projects currently has a $26.3 million balance.

APAH has owned and operated the building since 1997, when the county gave it a $475,000 loan to rehabilitate it. That loan was paid off in 2003.