The Arlington County Board yesterday unanimously approved the county's first life care facility -- apartment buildings for the elderly that include access to nursing and other services as a resident ages.

The facility -- two 17-story apartment towers atop a three-story base housing a community center and a clinic -- is part of The Ellipse, an office and residential development on North Taylor Street between Wilson Boulevard and Fairfax Drive.

The site is across the street from the Ballston Common shopping mall.

The life care facility is being built by the Marriott Corp. and construction is expected to get under way in the spring of 1989, according to a Marriott official. It will be the first such facility in Arlington, a county official said. Fairfax County and Alexandria have similar facilities, he said.

The Marriott project won praise from Arlington officials, who said the county is seeking a variety of facilities to meet the needs of a growing elderly population. "It's a welcome addition to the options available for older citizens," County Board Chairman John G. Milliken said.

The number of Arlington residents over 65 grew by 30 percent from 1970 to 1980, to 17,682 people. Planners expect that by 1990 there will be 21,840 elderly residents in Arlington, about 14 percent of the population.

The Marriott project, which is expected to take 20 months to complete, will have 350 apartment units for elderly people who can live independently, 32 rooms for those needing some medical or other supervision, and 50 beds for those who need round-the-clock care. Marriott has agreed to set aside five beds for Medicaid patients.

Marriott officials said some units will be rented. They are studying whether to sell others as condominiums with monthly fees, or to charge an entrance fee with monthly payments.