Mary Catherine Cramer's family moves every 18 months. That's why she made a point of becoming president of the parent-teachers association at Union Mill Elementary in Little Rocky Run, one of the largest new developments in Northern Virginia, with more than 2,500 homes planned.

Cramer's husband, Dick, is in charge of creating growth for an insurance firm, which explains why the family -- which includes three boys, ages 11, 9 and 5 -- moves so often.

"Being a transient, I first start by contacting the school system and having them send me test scores for different schools," Cramer said. "Then I ask {the real estate agent} to show me homes in this specific area that test scores are quite high in. I go for the most stable atmosphere I can for the short time I'll be here. I use the schools as the base for that."

The Cramer family is one of more than 1,000 trying to create a community out of Little Rocky Run, set on 900 once-wooded acres that now have mostly been carved up into quarter-acre lots. The development is off Braddock and Union Mill roads, just south of Centreville in western Fairfax County.

Eleven different builders are working there, creating homes that frequently feature traditional touches ranging from porches to Victorian towers, from two-story "Tara"-like pillars to copper sheathing on bay windows.

The average price of a town house is $130,000; detached houses sell for an average of $218,000. A large detached house on a full acre can go as high as $365,000.

Little Rocky Run is named for the stream that flows through the property. Some of the houses in the development boast a Clifton mailing address, despite the fact that they are just down the road from Centreville. Clifton is perceived by some as prestigious horse country and a Clifton address more "upscale" than Centreville.

David L. Hunter, Little Rocky Run's developer, said the tract -- located 45 minutes or more from employment centers such as the Pentagon -- was one of the last large undeveloped parcels in Fairfax County by the time he got to it.

Despite its location in "the boonies," Hunter said the development seemed like a good bet economically because of its proximity to emerging employment centers such as Dulles Airport, greater Leesburg, Reston, Herndon and Fairfax Center.

Little Rocky Run is not a planned development in the strict sense of a Reston or Burke Centre, which include master-planned shopping and commercial development to provide job opportunities.

But it is more than a simple subdivision. It not only has shared jogging trails, playing fields, tennis courts, tot lots and a recreation center with a pool (the first of three to be built), but also features a new Fairfax County elementary school that opened in September.

It also has a powerful homeowners' association -- a "shadow government" -- with committees and subcommittees that rule on such isues as land use, open space, the swim team and child care. And it is the school and the homeowners' association on which most of the community-building activities center.

"At these prices, you need to sell them more than just houses," Hunter said. "You've got to sell them community."

"We're planning a Saturday night sock hop at the recreation center to get adults out," said Diane Jones, chairman of the homeowners association's community activities committee. "We'll have a deejay; have people come dressed up in '50s style or we won't let them through the door. It's hard in an area where people are new and transient. It takes a few years before people get really active."

"Most of our friends come from within Little Rocky Run," said Cramer. "I met the majority at the school bus stop. School is the nucleus of our whole social gathering. Traveling dads are a normal thing around here -- the military, computer people, insurance. So you get couples to come over, and through people I have met, husbands get friendly, go play golf, end up coaching the soccer team or being an assistant coach for baseball."

Bob Roman, who heads the homeowners' association swim team, said that while the purpose of the team was to provide an activity for the kids, that "it does have that side effect of creating community because it takes close to 30 adults to run a meet." The team started with 35 children at the beginning of last summer. At the beginning of this season, he said, 140 had signed up.

But Roman's wife, Jane, stresses that the school is the focus of most community development.

"Parents work in the library. Other parents are helping get a publishing center started, publishing books of the kids' own writings," Jane Roman said. "Parents help with computers. They helped put on a musical called 'Peace Child.' "

The parents even threw a fancy art auction for the school that boasted a professional auctioneer and a crowd replete with dark suits and after-five dresses. "Families are moving in daily. That was one of the ways they found to meet other people," Jane Roman said.

Thirty-one percent of the buyers of detached houses in Little Rocky Run were relocating from outside the Washington area, according to the developer. Twenty-four percent of the town house buyers were similarly transient.

It's a young community with a large baby-boomer contingent. According to statistics collected by the developer based on the population that moved in during the first quarter of this year, more than half the town house owners are in the 24- to 34-year-old category, and there are few over 45. A similar majority of the single-family detached homes belong to those in the 35- to 44-year old contingent. There is hardly anyone in the development over 55.

Little Rocky Run is a family place. More than 98 percent of the detached-house owners are married, and 76 percent have children. In fact, in the detached-home area, there are almost as many children as there are adults -- 43.5 percent of the community residents are children. Thirty-six percent are between the ages of 6 and 11. Only slightly fewer -- 29 percent -- are between 2 and 5. There are more toddlers under 2 than there are teen-agers over 18.

In the town houses, only 28 percent of the population are children, and they are younger -- 46 percent between 2 and 5 -- typical of a younger up-and-coming adult population.

And up-and-coming they are. Among owners of detached houses, 44 percent of the families have annual incomes ranging from $60,000 to $80,000. Over 33 percent of the families make more than that. And 44.6 percent of the families have just one income, which implies that one parent is staying home with the kids.

Of the town house owners, 40 percent of the families have annual incomes between $40,000 and $50,000. But another 40 percent make more than that. Fifty-six percent of the town house households have only one working parent.

Overwhelmingly, the population is made up of professionals and managers, with only 10 percent of the houses being bought by military officers, and only 15 percent by government employes.