Officials from Arlington, Safeway Stores and Kaempfer Co. developers broke ground yesterday for a $42 million 12-story Rosslyn office building that will include a Safeway store when it is finished in mid-1987.

Last month, Safeway Stores closed its 24-year-old store at 1525 Wilson Blvd., the site of the new building, forcing customers, some of them elderly and without cars, to shop elsewhere.

Ernest Moore, public affairs manager for Safeway, said the company's lease was to run out in 1989, and by closing the store now Safeway was guaranteed space in the new building.

Officials from Safeway and Kaempfer, developer of the project, also took the occasion to dedicate a $15,000 silver Chrysler Plymouth van to carry Rosslyn senior citizens free of charge to Safeway's Arlington Cherrydale store during the next 16 months while the new building is under construction.

Safeway and the Kaempfer Co. plan to run the bus continuously during the daytime to Cherrydale and will split the cost.

They also will pay the $20,000 annual cost for the van's driver.

The shuttle service, which will be coordinated by the county's Economic Development Division, is expected to be operating by the end of the month.

John Tyers, director of marketing for Kaempfer, said the van is expected to leave from at least one location, the Wilson School at 1601 Wilson Blvd.

Moore said the new store will be about 31,000 square feet, about twice the size of the old one, and will include underground parking. It will be one story below ground, and include a pharmacy, a bakery, a deli-cheese shop and a plant shop.

Arlington County Board Vice Chairman Mary Margaret Whipple, addressing a crowd of about 100, praised the joint efforts of Safeway and Kaempfer. to preserve the grocery store in Rosslyn.

Whipple said the retention of the store was a "real achievement for this development" and called the preservation of grocery stores in general "highly important to the quality of life in Arlington."

The retention of supermarkets has become a major County Board priority in recent years as grocery stores have been displaced by redevelopment along the Metro line corridors.