When he appeared out of nowhere and announced his campaign last spring to unseat Prince William County's veteran Board of County Supervisors chairman, Sean Connaughton had just been jolted by disturbing news.

The assessed value of the $270,000, four-bedroom colonial he bought on Quantico Creek 10 years ago had plummeted by $50,000, a blow that Connaughton says propelled him into politics.

The region's economy was booming, but he and his neighbors were witnessing falling real estate values in a glutted housing market. The starter homes that have made Prince William a haven for young, middle-class families were, he concluded, choking the county and driving down real estate prices.

The 38-year-old conservative Republican lawyer from Triangle went on to build a winning campaign that asked voters why they should settle for problems created by unabated sprawl: stagnant home prices, bumper-to-bumper traffic, crowded schools spilling students into 140 trailers and the highest tax rate of any county in Virginia.

Like the victorious slate of slow-growth candidates in neighboring Loudoun County, he pledged dramatic change. Not so much slower, but smarter, growth. The new approach, he promised, would shift the county of 280,000 from a suburb of low-end town houses and strip malls on Washington's fringe to a place more like its wealthier neighbors, Loudoun and Fairfax counties. Prince William could then become a place for residents to work in rather than just commute from, with the healthier tax base paying for new streets and schools.

The message resonated. Even though he was a rookie so strapped for cash that he had to lend his campaign $45,000, Connaughton upset 23-year board veteran Kathleen K. Seefeldt (D) in the Nov. 2 election, winning by fewer than 700 votes. He was able to paint the 64-year-old Seefeldt as handmaiden to two decades of sprawl.

Connaughton, who will be sworn in as county board chairman today, says his mandate is not only to rein in growth, but also to continue efforts to turn around the down-at-the-heels image that has dogged Prince William for decades.

"I'll really feel I've been a success if people can say, 'I live in Prince William County, and I'm proud to live here,' " Connaughton said in an interview last week. "We don't have to be thought of as second tier."

Supervisor Ruth T. Griggs (R-Occoquan), elected in 1998 on a similar anti-sprawl platform, said many residents perceive the county and its government as passive witnesses to unchecked development. "We were essentially a rural county, and then we had a lot of things happen to us, as opposed to us making them happen," she said.

What's unknown is how fast the new chairman can change course--if at all. A slow-growth plan approved by supervisors last year slashed the number of houses that can be built in the county's rural western end, but officials have already granted zoning approval for 40,000 new homes.

To wipe some of the unbuilt houses off county planning maps, Connaughton proposes offering builders incentives to switch to commercial projects or pricier homes. But no one knows whether such tactics will succeed.

Overbuilding has flattened the value of both low- and high-end homes. When it's time to move out, homeowners are more likely to hold on to their property and rent it out, instead of selling. Prince William's already booming rental market has resulted in an influx of transient families that require more services. At the same time, the county's growth has meant increasing crime.

Unlike the slow-growth activists elected in Loudoun last month, Connaughton will be one voice on a county board that hasn't always been comfortable with dramatic change, and which has been intensely loyal to Seefeldt.

"Prince William County is probably in as good a shape as any county in the nation and much better than most," said Supervisor John D. Jenkins (D-Neabsco), a Seefeldt loyalist and now the board's longest-serving member.

"The board has been addressing development and has a proven track record," Jenkins said, describing Connaughton as "wet behind the ears."

Connaughton, who will preside at his first meeting Jan. 4, is the first to acknowledge he's an unknown quantity. "People might say I'm too young to be doing this," he said.

Connaughton is among the region's youngest elected local officials, which could give him a leg up in understanding the needs and problems of the young families that have nearly doubled Prince William's population in the last two decades. His own children are 5 and 7, and he will be the only board member with a daily commute to the District along Northern Virginia's traffic-logged highways.

A Naval Reservist and former merchant mariner, Connaughton moved to Prince William with his wife, Teresa, in 1987. He was already familiar with sprawl, having grown up in the most famous subdivision of them all, Long Island's Levittown. His father was a Nassau County, N.Y., police officer.

Since his election, Connaughton has drawn praise from business leaders for his plan to end the tangle of regulations and hefty fees that he says delays permits and thwarts commercial projects.

At the same time, his support for a controversial 60-mile outer beltway through Prince William has rankled slow-growthers, who fear it would generate more sprawl. Like the new Loudoun board members, Connaughton says he believes counties have more power to limit development than they might think. He favors testing that authority by asking developers for more money to finance roads, schools and other services, and by stopping projects in areas without adequate roads and schools. "We've just never had the political will to do it," he said.

Connaughton and Scott York (R), Loudoun's slow-growth chairman-elect, have met to discuss their mutual concerns. Connaughton, York said, "had a presence about him that will help him lead the county."

But Prince William struggles with issues that its neighbor does not, among them, a tax rate of $1.36 for each $100 of assessed valuation, a household income 5 percent lower than Loudoun's, and a growth pattern that brought an explosion of cheap housing before a viable commercial tax base was in place. High-tech firms are only starting to arrive.

These days, Connaughton is paring back his maritime law practice, which had him before the U.S. Supreme Court last week arguing against environmental regulations for oil tankers. Some political observers are already speculating on where he wants to go from here, but his political future is one subject Connaughton declines to discuss.

Friends and colleagues say a run for Congress is likely, especially if redistricting after the 2000 Census brings a new congressional seat to Prince William. "If he wants a career in politics, Sean has every opportunity to use this as a launching pad," said one GOP colleague, Supervisor Edgar S. Wilbourn III (Gainesville).

Sean Connaughton

In Profile

Age: 38

Address: Triangle, Prince William County.

Education: U.S. Merchant Marine Academy (bachelor's degree), Georgetown University (master's degree), George Mason University (law degree).

Background: Raised on Long Island, N.Y. Moved to Prince William County in 1987. Naval reservist.

Job: Lawyer with D.C. firm of Eckert Seamans Cherin & Mellott

Family: Wife, Teresa, children Sean, 5 and Courtney, 7

Favorite Sunday pastime: Fishing with his children.

Favorite restaurant: Capitol Grille in D.C.

Last movie seen: "Saving Private Ryan"

Last book read: Nations at War: A Scientific Study of International Conflict, by Daniel S. Geller and J. David Singer

Hobbies: Lifting weights, running