The Arlington County Board, hoping to stem the rapidly dwindling number of supermarkets in the county's high-rise development corridors, approved a plan yesterday for a 12-story Rosslyn office building that calls for a major grocery store on a lower level.

The board voted unanimously in favor of giving the developer extra building density in return for the firm's attempts to ensure that a supermarket would be included in the $45 million project in the 1500 block of Wilson Boulevard, where the Rosslyn Safeway currently stands.

As a condition of its approval, the board insisted that the developer, the Kaempfer Co., must come back to the board for approval of any change in the use of the almost 30,000 square feet set aside for a supermarket.

Under the caveat, the firm can use the space only for another parking level if a lease cannot be signed with a supermarket, or for other neighborhood-oriented retail stores if a supermarket does not sign a long-term lease. The store would be at ground level on one end and the first basement level at the other.

The retention of supermarkets has become a major board priority in recent years as grocery stores have been displaced by massive redevelopment along the Metro-stop development corridors where the county is trying to encourage residential growth to offset the preponderance of office buildings.

Only two supermarkets remain in the Rosslyn-Ballston Metro corridor -- the Virginia Square Giant and the Rosslyn Safeway -- and both are threatened by planned developments that would leave fewer neighborhood-oriented stores to serve more residents. A third supermarket in the corridor, the Clarendon Grand Union, closed earlier this year.

"Please press as hard as you can for a better guarantee of a food-service store within walking distance of three moderate-income neighborhoods," said Barbara Nnoka, one of several Rosslyn area residents who noted how hard the loss of a neighborhood supermarket would be on the elderly population.

J.W. Kaempfer Jr., president of the development firm, said negotiations are under way with Safeway, which has just under five years remaining on its Rosslyn lease, to persuade the food chain to relocate in the new building.

But noting Safeway's lease rights and the pressure on the county to keep a supermarket there, Martin D. Walsh, Kaempfer's attorney, said, "Every time Safeway thinks they're more in the driver's seat, their negotiations harden."

No Safeway representative was at the meeting, but Tom Castleberry, director of the firm's local real estate division, said in a Friday interview that the firm will not break its lease early unless it can be included in the new building.

He said the planned basement location for the store made it a "risky venture" because customers would have to park in one of 59 underground spaces.