John T. (Til) Hazel, the influential lawyer and developer who has masterminded some of Fairfax County's largest and best-known developments, has proposed a 542-acre commercial and residential project in Prince William County -- his development firm's first major project in Washington's outer suburbs.

"Fairfax is filling up and land prices are high," Hazel said in an interview yesterday. "Obviously the next step is into the adjacent counties. That's our own back yard."

Official said Hazel's multimillion-dollar project, called William Center, could speed economic development in Prince William, already second only to Fairfax County in population in Northern Virginia.

"Prince William is starting to explode," said Herbert S. Miller, president of Washington-based Western Development Corp., which recently opened a large discount mall there. "I think it's the future of the Washington area."

The site of Hazel-Peterson Co.'s project is a tract near Manassas, owned by the Marriott Corp. and once planned as the site for a theme park. The land, believed to be valued at $9 million, is bracketed by I-66 and Lee Highway (Rte. 29), just west of Manassas National Battlefield Park.

It is about 10 miles west of Fair Lakes, another large residential and commercial project Hazel-Peterson is building in western Fairfax.

Hazel proposes to convert the Prince William land, now a sweeping vista of vacant farmland, into an office park of about 275 acres, a residential neighborhood of 975 town houses, garden apartments and single-family homes and a small neighborhood shopping center. The project would take at least 10 years to build.

Hazel would not set a figure on the project's value, but development officials said it should cost more than $100 million.

Some developers also noted that the proposal was typically shrewd in Hazel's choice of timing and location. They noted that another Fairfax developer, Centennial Development Corp., had raised hopes in Prince William last year by proposing a high-technology office park on the site, but backed out in December when the investment costs appeared too high.

That dashed the hopes of many in Prince William who see the Marriott tract as a chance to lure big-name corporations and broaden the tax base of the fast-growing county.

Prince William Supervisor G. Richard Pfitzner described the land as "the linchpin of the economic development of the I-66 corridor that hopefully will be the standard that all our other industrial stuff will be compared to."

The Hazel-Peterson proposal also capitalizes on as-yet-unfunded plans by Prince William officials to build a Manassas bypass that would sweep through the county's west and form an interchange with I-66 next to William Center. The interchange, county officials acknowledge, would be worth millions of dollars in rental fees and home sales to whoever develops the tract -- a fact that Hazel had in mind when he negotiated a contract on the land.

If Prince William waits for state funding to build the interchange, county officials said, it could take more than a decade. They said that a key condition for the county board's approval of Hazel's proposal may be a significant contribution from him toward construction of the interchange.

Hazel said he was not interested in helping to fund an interchange. "We frankly don't need an interchange with 66," he said.

County officials, worried that the proposed residences would force them to provide costly new schools, libraries and police and fire stations, also said they would try to persuade Hazel to cut down on the amount of residential units he has proposed. County officials calculate that the site is perfectly suited to office buildings, which produce more net tax revenue and require far fewer government-funded services than do residential neighborhoods.

Under pressure from Hazel-Peterson, the county last month amended its zoning laws to permit developments that combine commercial and residential development. Frank McDermott, an attorney for Hazel-Peterson, told the County Board that the developer would "walk from the site" if the new zoning was not approved. The board then adopted the category, 6 to 1.

"It's totally absurd for us to go in there with nothing but a corporate office park and nothing around to ripen the interest in the office park," Hazel said, adding that he hoped the center would be "a catalytic agent" for a commercial boom in the county.