Private developers in Northern Virginia increasingly are moving into a market traditionally left to nonprofit groups and charities -- housing for the elderly.

But it is elderly housing with a difference. Developers are proposing full-service retirement centers known as life-care communities.

The latest project to be unveiled in Northern Virginia is for a retirement community on 142 acres just west of the historic town of Occoquan. Weaver Bros. Management Corp., a Bethesda-based company, filed a rezoning application last month with Prince William County for 500 elderly housing units and up to 150 medical beds on land now zoned for one house per 10 acres. As a life-care community, the project would offer residents a range of housing, recreational facilities, living assistance and medical care in one setting, allowing retirees to move into a single community that can provide for most of their needs until death.

Weaver Bros. is known in the county for its residential community, Lake Ridge, developed by its Ridge Development Corp. subsidiary.

Weaver Bros.' project, if approved, would be the second privately built life-care community in Northern Virginia. Last year, Marriott Corp. joined hands with the Reserve Army Retirement Foundation to build a similar project for ex-military personnel near Fort Belvoir in Fairfax County. A life-care community also was approved for the Manchester Lakes residential development in southern Fairfax County, but that development has been beset by financial difficulties.

Northern Virginia's four existing life-care communities -- three in Alexandria and one in Fairfax City -- are run by church-affiliated groups, and a fifth proposed by Fairfax Hospital Association for Reston would have a not-for-profit tax status.

Mark Epstein, assistant director of Health Systems Agency of Northern Virginia Inc. (HSA), said statistics showing the rising number of elderly in the area have helped persuade the private sector to tackle this market.

"It's certainly one of the fastest-growing segments of the community," Epstein said.

HSA projects that the number of people 65 years and older in Northern Virginia will increase by 49.3 percent between 1980 and 1990, to 97,100, or 8 percent of the region's population. The 1980 census showed 5.9 percent of the region's 1.1 million residents over 65 years of age.

In addition, these people are relatively affluent. Husband-and-wife households in Northern Virginia have a median income of $24,620 a year, approximately double the national median, according to statistics compiled by the Northern Virginia Planning District Commission. Median means that the same number of people earn more than $24,620 as earn less.

The statistics mean that many area retirees can afford the hefty entrance fees and rents that life-care communities usually charge. The Washington House in Alexandria, run by Temple Foundation, a Methodist-affiliated group, has the cheapest life-care rents in the area: $670 for a single person in a studio apartment plus a "substantial" entrance fee, according to a guide compiled by The Fairfax County Area Agency on Aging. The Virginian, outside Fairfax City, also run by the Temple Foundation, has the highest rents in the area, charging $1,830 a month or more for a couple in a two-bedroom unit.

These figures, along with surveys that showed the vast majority of retirees prefer to remain in their geographic area, encouraged Weaver Bros. to go ahead with plans for its life-care community, said Kenneth Thompson, a principal in the company.

"It's a myth that they sell their cars, pack up their homes and move to Florida. They want to stay close by," Thompson said.

The Washington area also is a good market for private life-care centers because it offers retirees many cultural and recreational opportunities that "eluded them in the hectic working years," added Anthony Sala of Ridge Development Corp.

Though many retirees -- an estimated 95 percent, according to experts on the elderly -- remain in their neighborhoods after their children have left home, many want to rid themselves of the burden of maintaining a large home, Sala said.

Like similar facilities, Riverboat Life Care Community -- the name Weaver Bros. has chosen for its development -- would provide a variety of living arrangements. The development, to be located between Occoquan and Lake Ridge, would have 200 small detached homes called cottages and 300 apartment units, all of which would be easy to maintain, Thompson said. Residents either could purchase or rent their units, he added.

Yard, house and meal service would become available to residents as they grew more feeble, Thompson said. When residents required skilled nursing care, could utilize the 100 to 150 medical beds proposed for the community.

Until then, residents could make use of the community's recreational facilities. A general development plan filed with the rezoning application shows a pond, two recreation areas and a recreational building on the property. Sala said it is too early to say what type of recreation might be provided. Weaver Bros. also is waiting until the rezoning application has been approved before contracting with a medical organization to run the nursing home beds and the 24-hour-a-day medical emergency service.

Riverboat is seeking approval for the project under a rarely used zoning category in Prince William, Residential Elderly and Handicapped. County planning director Roger Snyder said reviewing the Weaver Bros. application will present a challenge to his staff since it has no criteria such as normal traffic patterns by which to judge a retirement community.

"We don't want electric golf carts going up and down Davis Ford Road," he joked.

Weaver Bros. also would have to obtain a certificate of need for the nursing home beds -- frequently hard to come by -- from the state health commissioner. That application may be eased, however, by a projected shortage in the number of beds; Fairfax County's Health Care Advisory Board estimated in a 1984 report that Northern Virginia will have a shortfall of 957 nursing home beds by 1990. The area today has a total of 4,130 nursing care beds.

The existing life-care communities in Northern Virginia offer 213 skilled nursing care beds, and 974 beds for people needing minimal assistance in daily living.