When Centennial Development Corp. President Pete Scamardo looks down from the top of his new six-story Executive III office building near Dulles Toll Road, the Reston he sees around him is vastly different from the Reston of 1978.

That was the year Scamardo, then an adventuresome, budding developer, made his decision to develop his first office building in Reston. In those days, the major commercial space was concentrated at Reston's own International Center and in the U.S. Geological Survey buildings.

Now, Centennial, the largest single developer of commercial space in Reston, has more than 750,000 square feet of office space in Reston, and another 500,000 is planned for construction in the next 18 months. The 21-year-old new town itself has blossomed into a major office market.

Reston now boasts more than 6.5 million square feet of office and industrial space that houses a heavy concentration of high-technology firms. Nearly one-third of that space has been built in the last three years. Reston has attracted a residential base of well-educated professionals, providing a substantial reservoir of potential employes who have helped lure businesses to the office spaces now available or planned.

Reston apparently has achieved much of the dream originally envisioned by its creator, Robert E. Simon, in the early 1960s, according to corporate tenants, residents and county planners. It has become not only the carefully planned mixture of residential, commercial and recreational space, but also a mecca for some of the giant, as well as fledgling, corporations seeking office space in the Washington area.

J. Hunter Richardson, Reston Land Corp.'s marketing vice president, and developers said Reston's one main drawback for corporations and homeowners alike is its lack of restaurants. But Richardson said he hopes that the current diversification into office space will help lure more quality restaurants to Reston.

Reston today has more office space than did Tysons Corner only five years ago, Richardson said. More than 329,000 square feet of office space was leased in Reston in the first six months of this year, making it second only to Tysons Corner as the most-sought-after market in Fairfax, according to county Economic Development Authority statistics.

Industry statistics indicate Reston now has an office vacancy rate hovering just above 8 percent, about one-third lower than the countywide 12 to 13 percent vacancy average. County officials predict another 1.4 million square feet of office space in Reston will be under construction by the end of December.

Growth has been "overwhelming and staggering," according to Paul Wyatt, manager of the Piedmont Airlines reservations center, which opened in Reston in 1981 to serve customers from New York to North Carolina. By the end of 1986, Piedmont will have 500 workers at its Reston facility.

The growth in commercial space has created the basis that today forms one of Reston's selling points: its reputation as a high-technology community with a built-in residential base needed to support businesses. The planned-community concept, coupled with a strong commitment by Fairfax County government officials to abide by the long-standing development plan for Reston, has given those thinking about moving corporations to Reston insurance about the community's future.

The opening of the Dulles toll lanes last year has given Reston almost unsurpassed accessibility to most parts of Northern Virginia, the western Maryland suburbs, the District and National and Washington Dulles International airports, according to commercial real estate brokers.

Tom Jones, manager of engineering services for General Electric Space Systems division, said his firm consolidated several Crystal City offices by moving to Reston because "we were looking for possible places in a high-technology community."

"The decision to move to Reston was also partly made because of the demographics of the highly educated work force. We are great consumers of software programmers and systems engineers," Jones said.

J. David Hann, president of GTE Telenet's world headquarters in Reston, said GTE moved from several different spots in the Tysons Corner area.

"We thought Reston would be a better location for a high-technology company to attract high-technology people," Hann said. "We were concerned when we moved. It's about 11 miles further out from Washington . But we have had no negative reactions from employes ."

GTE's four-building, campus-style, 47-acre headquarters overlooking the toll road includes sports fields and a health and fitness center for employes.

Hann said the toll road played a major factor in GTE's decision to move to Reston, a factor echoed by others making recent decisions to move there. Easy accessibility to both airports is another major lure, brokers and corporate leaders said.

"One major reason a lot of commercial tenants are heading to Reston is the opening of the toll road," said Phil Meany, a broker with Leggat-McCall. He said tenants moving often are looking for space that can be expanded to accommodate future company growth. That sort of space, available in Reston, is hard to find in densely developed areas such as Tysons Corner, Rosslyn and Crystal City, brokers said.

Jack Griffin of Mulligan/Griffin & Associates, a development and investment firm specializing in high-tech space, said high-tech tenants generally want a professional image, but "insist on avoiding the 'Taj Mahal' image in their office environment." He said many of the buildings in Reston, including the GTE facilities that his company developed, meet this "standard of Spartan elegance."

Even though Reston advocates praise its high-tech image, the market now is expanding to include a broader range of tenants, brokers and developers said.

The Federal Home Loan Mortgage Corp., which now has its headquarters in the District, soon will move into the first of twin buildings it will occupy near Weihle Avenue and the toll road. The first Freddie Mac office tower, which was built by Sammis Corp., features a dramatic atrium, and the structure is considered symbolic of the emergence of a more intense attention to architecture in Reston.

Centennial Development's Executive III building is a brick-and-glass, six-story structure complete with the upscale office amenities such as elegant lobbies and brass fixtures that can be found in many new District and Tysons Corner office buildings.

"We are now moving into an extremely sophisticated user market," said David A. Ross, Centennial's vice president of marketing.

Allstate Insurance is relocating sales and claims representatives from several locations, including Roanoke, to Executive III next month. Jerry Allen, Allstate's field real estate manager, cited the "total community, the county school system's good reputation, proximity to the toll road and the Class A building" as reasons for the move.

But the centerpiece of Centennial's work in Reston is expected to be the 280,000-square-foot Summit at Reston project designed by the internationally acclaimed Architect's Collaborative. Its sculptured terraces and three-story glass atrium will dominate the complex, which is scheduled to open on Sunrise Valley Drive next year.

Scamardo said he decided to move to Reston very early in 1978 because "I believed in the initial concept, its planners, its developers and its future."

He said he has developed his huge holdings based on a system that may "not be the most cost-effective way to build speculative office buildings" by paying "close attention to each and every detail that goes into the development of a project from its planning through its completion and its occupancy."