Groundbreaking for an 112-unit condominium complex for people 55 and older is expected to take place next spring on a site just a few hundreds yards from the Vienna Metro station.

The age-restricted development is the first of its kind in Fairfax County.

"We are real excited about it," said Will Collins, president of the Concordia Group in McLean, land developers for the 4.75-acre site. Collins said one of the main selling points will be the apartment complex's proximity to Metro. He said though the station is a short walk away, shuttle service will also be provided between the development and Metro.

Developers are trying to tap into a demographic bubble that is about to expand. The first gray wave of the post-World War II baby-boom generation is turning 55 and, in the next 10 years, it will represent about 35 percent of the county's population.

"You are going to see a big bulge in the age pyramid for that generation," said Anne Cahill, the county's demographer, referring to people born between 1946 and 1964. "That's a national trend, not just local."

Porten Homes of Rockville is building and marketing the $26 million development, called Saintsbury Plaza.

"We want to make it very much an upscale condominium complex," said Ray Sobrino, vice president of development and sales for Porten Homes. "We see a big market for older people who want to stay in the area. We see a big opening in that market and being close to the Metro helps."

All the units will be two- or three-bedroom apartments, which will average about 1,450 square feet. Sobrino said the company wants to build units that will feel like single-family homes on the inside while the outside is owner-maintenance free. Porten is looking to sell to people whose children are gone, are still working and want to trade in their single-family homes and all the associated chores for a condominium next door to Metro.

"This is not a nursing home," said Supervisor Gerald E. Connolly (D-Providence). "It isn't even an assisted care facility. This is the kind of housing that, as the population ages in the county, will allow all those people to stay here in the county and not retire to Florida. . . . As the population ages, we are going to need housing choices in the county."

The cost of the units will range from $270,000 to $370,000, according to Lisa Cogley, vice president of sales and marketing for Porten. "We looked at other sites," she said, "and it came down to the market study, the kind of marketplace and certainly the proximity to the Metro."

The developers are betting on the prospect that the baby boomers want to give up their yard work but not the feeling of having a nice yard. So 1.5 acres on the site will remain in its parklike state, according to Collins.

"This is a project that we are really proud of," said Collins. "It is an example of smart growth."

Still, there is some risk in this and similar future projects that the population group they are targeting may move.

"You can't control those things," Cahill said. "We are projecting that our senior population should grow, but some will move away and some will not. We don't know how many will move away."

Connolly said it is all about options. If there is housing available for people who want to stay near their children and family and friends after they retire, then that may influence their decision.

"We don't have a lot of housing options for folks as they turn grayer and become empty-nesters but still want to have productive lives and want to play a role in the community. They want some options," said Connolly.

The units will go on sale in the spring. The first condominiums will be ready for delivery at the end of next year, and the project is expected to be completed by the end of 2004.