A nonprofit foundation that specializes in providing retirement homes with built-in "life care" medical facilities wants to construct such a project in Arlington County, where the elderly population has increased 30 percent in the past decade.

The Rosslyn-based Temple Foundation Inc. is proposing a $23 million facility near the county courthouse, a Metro subway station and a planned multimillion-dollar mixed-use Courthouse Plaza development. It would have 180 residential units and 74 hospital beds for people more than 65 years old.

County residents in that age bracket now account for nearly 12 percent of Arlington's 152,000 population, according to the 1980 census. Their numbers are growing so rapidly that the County Board last spring held a special community forum on problems of the elderly.

"A lot of people are in the age bracket where they have to consider seriously what they're going to do," said Ken McFarlane Smith, the foundation's lawyer. "Many of them want to stay in Arlington and we could offer them an opportunity to stay right in the county, where the action is, where they've lived most of their lives."

Unlike conventional nursing homes or retirement complexes, the project is designed to allow healthy, ambulatory people to pay a "founder's fee" to get into the project, plus a monthly fee that covers housing, food, entertainment and medical care costs should the patient need to be transferred to the medical wings. The resident, however, does not own his or her apartment.

"If you have a couple in an apartment and one of them gets sick, everything is right there," Smith said.

But there are two major hurdles facing the project, which will come before the County Board at its meeting Saturday: winning permission from state health officials for the hospital beds and getting the board to approve the sale of $23 million in industrial development bonds to finance the project.

Board member Mary Margaret Whipple said the project "might be an appropriate use" of such bonds, which are used to attract projects considered an asset to the community.

"You can't do this with conventional financing because the interest rates will kill you," Smith said. But, he said the foundation's two other operations--the Virginian in Fairfax County and Washington House in Alexandria--received local funds. The foundation was formed by members of the Temple United Methodist Church in Rosslyn.

The Arlington project, which would be known as the Jefferson House, would be the first of its kind in the county, where there are currently three retirement complexes and three nursing homes. It would be located on a two-acre parcel bordered by Fairfax Drive, N. Cleveland, N. Barton and a portion of N. 15th Street to be built. The tallest part of the facility would be 10 stories on the 15th Street side, tapering down to six stories on Fairfax Drive.

John S. Showell, president of the Courtland Civic Association, in whose neighborhood the facility would be built, said the group has not voted on it, but that "there is a very strong consensus in support of it . . . . We prefer it to a series of stacked houses."

The project, which the planning commission and the county planning staff have endorsed, would have 114 one-bedroom units and 66 two-bedroom units, each with its own kitchenette. Smith said it would also have communal kitchen and dining facilities, a recreation room, exercise and therapy room, auditorium-theater, two nondenominational chapels and a beauty shop. Two meals would be served in the dining room daily.

Smith said initial cost projections put the average price for a one-bedroom "founder's fee" at $75,000. In addition, the average monthly fee for such a unit would be about $725 for rent, medical care, food and entertainment. "They are purchasing the use of the apartment for life," Smith said, adding that the apartments cannot be willed to heirs.

The "life care" medical facility, with private and semiprivate rooms, would have round-the-clock nurses and physicians there in the daytime and on call at night. Applicants must be able to care for themselves when they move in.

"We're not holding ourselves out as a hospital," Smith said, adding that seriously ill patients or those requiring surgery would have to go to a regular hospital for treatment.

The project could open in mid 1985 if the County Board approves the foundation's rezoning, land use and site plan requests.

Whipple said she is concerned about what develpment may occur there if the board rezones the property and the foundation's plans fall through.

"We need retirement homes and different options for living arrangements for the elderly . . ." she said. "I would hope that this is one of what would be a whole series of options for the elderly."