It took Tom and Pat Gaul a year and a half in a terrible market to sell their old house in Richmond, but they finally found a buyer this winter and now, in turn, have bought their own new home in Prince William County.

"We've been renting all this time," Tom Gaul, principal of the county's Woodbridge High School, said last week as he surveyed his half-built house. "It's great to see the market pick up so we can buy our own home again."

The Gauls are not the only new home buyers in Prince William. The county is in the midst of a housing surge, with major county homebuilders reporting sales increases of 10 to 40 percent over last year at this time and with the county reporting a whopping 224 percent increase in the number of applications for the construction of single-family homes, town houses and duplexes.

Real estate brokers say there is a building boom because the average home in Prince William costs $10,000 to $30,000 less than those within the Beltway, luring young couples and families down I-66 and I-95. Developers are eager to build in Prince William County, approximately 30 miles south of the District, because of the demand for houses in the area, brokers say.

The surge of new home buyers is made evident by the numbers of people who have been touring the dozens of model homes in the county's subdivisions, according to builders.

"Some afternoons we have 40 or 50 people looking at the home at once," said Judy Byrne, a model home hostess in the year-old subdivision of Lynnwood. "We have people wanting to move up from town houses, young people wanting to move out from the District. It is just incredible."

The increase in house sales will mean more tax revenue for the county, officials say. But the new homeowners also will create a heavy demand on the schools, and police and fire protection needed to service their communities.

And unfortunately, the average household will cost the county more in services than the county will receive from the household in property and sales taxes, the officials say.

"You could call it a mixed blessing," said County Assessor Ben Kelsey last week. Kelsey already has told the County Board of Supervisors that the housing upswing will bring another $1.6 million this year in unanticipated tax revenue, prompting the supervisors this week to cut the property tax rate from $1.42 to $1.39 per $100 of assessed valuation.

Supervisor Chairman Kathleen K. Seefeldt said she is happy the increased revenue allowed supervisors to propose the promised tax cut, but she said the new residents also will create conflicts in the county. "Transportation is our most pressing problem," she said. "The new housing will put a burden on the state roads, and they are already overcrowded."

Seefeldt said the county anticipated the increase in home sales. In fact, she said supervisors have spent the last 10 years refining master plans and setting policy in an effort to cope with a continual housing boom.

"There was a lull in 1980-'81," said Seefeldt. "But even then we didn't experience the downswing that some of our neighbors did. There is a tremendous pent-up demand for housing, and we expect to see quite a surge in our direction."

Prince William County is not alone. The U.S. Department of Commerce reports that construction was up 15 percent in Northern Virginia last year from the year before. And Fairfax reports a 300 percent increase in the number of building permits issued in the first three months of this year compared with the same period last year.