The new town of Reston is not so new anymore. In fact, there's just one new detached house left for sale in the community.

"It's coming as a shock to some people, but we actually are running out of land," said Thomas J. D'Alesandro IV, vice president and eastern regional manager for Terrabrook, the fourth developer of the planned community in western Fairfax County.

Reston today has about 60,000 residents, far fewer than the 80,000 anticipated when Robert E. Simon Jr. founded the architecturally acclaimed town in 1964 and used his initials to give it a name.

With family size decreasing markedly in the intervening 3 1/2 decades, the community's population likely will peak at something a bit higher than it is now. Eighty town house sites and 250 condominium units are yet to be developed.

The last detached houses would already have been built, but D'Alesandro said the county fire department decided it did not need a fire station that had been planned where Reston and Wiehle avenues meet in the northwest corner of the 7,400-acre community, so the three-acre site was sold to Gulick Group Inc., a home builder.

Gulick has two traditional-style homes under construction at the site, which it calls the Estates of Highland Park. It has contracted to build two other adjacent houses and anticipates selling the fifth, and last, home site within the next month.

All five houses are base-priced from $444,900 to $550,000, although they usually sell for more after buyers select options.

"It's the end of an era -- not only locally, but nationally," said Dean Yeonas, Gulick's general manager. "Reston is recognized as one of the early communities that went forward with some vision and large-scale planning and pulled it off with some success."

There are about 21,000 housing units in Reston, about 4,800 of them detached houses worth between $200,000 and $800,000, according to D'Alesandro. Reston is also home to nearly 7,800 town houses, about 7,400 condos in apartment-like buildings and more than 700 units in elevator buildings.

While housing construction in Reston nears completion, development of the commercial-business side of the community remains a work in progress. About 40,000 employees work at 2,300 businesses in the town, many of them in large high-tech companies that have built or rent space along the Dulles Airport Access Road and Toll Road.

While some office space is still being built along the airport corridor, most of the office construction is occurring or planned for Reston Town Center, the commercial core of the community. It contains upscale shops, a Hyatt hotel, a skating rink and several restaurants. Typical suburban shopping centers -- large, boxy buildings next to huge parking lots -- have been built a short distance away along Reston Parkway.

Terrabrook and Boston Properties Inc., a major office developer in the Washington area, are finishing One Freedom Square, an 18-story office building along Market Street, the town center's main street, and are planning to start a second building of similar size in a year. In all, D'Alesandro said about 2 million more square feet of office space might be added to the 13 million already built in Reston.

"We'll finish in about three years," D'Alesandro said.

As Reston nears completion, many in the community cherish what it has become. Yet they voice some regret about various aspects of the landscape and some anxiety about life there as the community approaches middle age.

John H. Thillmann said he first discovered Reston when he drove through the community on the way to college. He has lived there for 28 years now. For a while he ran the county zoning office, later served on the Fairfax County Planning Commission and now is a vice president of a development firm.

He also is chairman of the Reston 2000 Task Force, a community effort that last year studied how the town might change over the next 20 years. The task force is considering how to implement a vast array of recommendations it made in a November report.

"We're a mature community. We achieved our maturity because our parent left," Thillmann said, referring chiefly to Mobil Corp., which followed Simon and then Gulf Oil Corp. as developers of Reston. "The developer was always in a position of telling us what was coming in the next couple years."

Now that the community is nearly built, Thillmann added, "We're still trying to figure out how to behave, develop political leadership, decide how we interact with our neighbors. We don't have a developer to bash anymore."

It is an assessment shared by D'Alesandro and Simon.

As D'Alesandro said, "We've built a place, a defined community like Arlington, Cleveland Park or Takoma Park. It's a model for how to preserve the environment but also accommodate our new businesses."

Simon, now 85, was the community's founder and first developer, from 1961 to 1967. But after he got Gulf to invest in the fledgling community, the oil giant ousted him. He said he lost $1 million on his venture. Five years ago, he returned to Reston, to live in a 13th-floor condominium at Heron House overlooking Lake Anne, one of four lakes in the community.

He is most disappointed with the way developers who succeeded him built Reston's shopping centers. "It's where cars gather," he said, rather than the people-oriented village centers he envisioned.

Such shortcomings aside, Simon likes what he set in motion at Reston.

"I hate this business of being such a sourpuss," he said. "I am not a bitter old man. I am a happy old man. When you're here, you know you're in a special place. I'm happy to have had a part in it."

CAPTION: Robert E. Simon Jr., founder of Reston, lives in a condominium overlooking the town's Lake Anne. Now 85, he says he is generally happy with the town.

CAPTION: Construction of new housing is nearly complete in Reston, but new offices are going up at Reston Town Center.