Patricia A. Petitt packed up and moved to eastern Loudoun County a decade ago, leaving her Tysons Corner home, she recalls happily, for "lots of trees and open areas."

Now she's spooked. Developers are planning a half-dozen major office and industrial parks in eastern Loudoun. Several huge residential projects are in the offing. A mammoth plan by Xerox would bring an estimated 30,000 jobs to the area. And Virginia Gov. Charles S. Robb's planned Center for Innovative Technology also is expected to attract thousands of new residents and employes.

Last week, Pan American World Airways announced plans to use Dulles International Airport on Loudoun's southeastern border as a hub for its domestic and international flights to Europe and South and Central America, starting June 1. Dulles and Loudoun officials said the move would add as many as 10,500 passengers a week to the airport, inject as much as $100 million annually into the area's service economy and lure new business headquarters to the Rte. 28 corridor.

"It's all gone wild," Petitt said. She and her husband are looking to move west again, another shuffle that she hopes will put them beyond the reach of the metropolitan area's westbound sprawl.

Fueled by the proximity of Dulles, the recently completed Dulles Toll Road, affordable land and the shortage of available large tracts elsewhere in Northern Virginia, a development explosion that planners say will transform landscapes and life styles already is causing tremors in eastern Loudoun County.

Local politicians say the county's public debate for the coming decades will be dominated not by the question of whether Loudoun should grow, but by the contours that growth should take.

Growth is all but unknown in much of the county. In western Loudoun, farms stretch for hundreds of acres across the foothills of the Blue Ridge Mountains. The meadows are laced with white fences. In the horse country that surrounds the hamlets of Round Hill and Hamilton, residents consider anybody who lives within three or four miles a neighbor. Visitors from Washington make the hour's drive on weekends to breathe the air and gawk at the cows.

Eastern Loudoun has a different feel. Developers have a few bridgeheads amid the wide-open farm land. Housing for the middle class is under way.

Planners say the coming boom in Loudoun will be concentrated in the eastern end, in part because that area has substantial tracts of land zoned for industrial use and equipped with water and sewer lines. Most of the major projects are clustered along two main roads: Leesburg Pike (Rte. 7) from the Fairfax County line to Leesburg, and Sully Road (Rte. 28) from Rte. 7 to Dulles Airport.

"Loudoun's like a little snowball at the top of the hill that somebody just gave a nudge," said county planning director Frederick P.D. Carr. "We're really the next frontier."

The snowball already is gathering momentum. Loudoun is the only jurisdiction in the metropolitan area whose population is projected to double over the next 25 years -- to 140,000 from the current level of about 70,000.

That means the county will be growing by an average of 2,800 people every year until -- and probably for some time after -- 2010. The school system likely will double as young families seeking first homes settle in new neighborhoods. Taxes, at least in the short term, are likely to rise along with increased demand for services and new schools, county supervisors acknowledge.

The projected growth in office space also is startling. Today, all of Loudoun's offices would fit inside a project the size of Fairfax County's Fair Oaks Mall. But each of a half-dozen office projects either proposed or approved in the eastern end would at least double the amount of office space in the county today. The Xerox project alone calls for up to 20 major corporate headquarters and a dozen smaller office buildings.

"We've had days when everyone's on the phone and people have been lined up out the door," said Sara Howard-O'Brien, the county's land development chief, who handles inquiries about new office projects. "We're treading water about six feet under. It's very difficult to keep track of all that's going on."

Publicly, county politicians hail the coming boom and call eastern Loudoun the economic breadbasket of the county. But while office parks and factories mean a substantial profit for the county at tax time, new homes and apartment buildings -- which must be serviced by extra fire and police protection and new schools -- mean more spending.

Taxes will climb, officials say, unless commercial and industrial growth outstrips residential growth. Predictably, county politicians and bureaucrats say they favor commercial growth, but they concede there is little they can do to forestall residential growth without painful -- and probably unsuccessful -- court battles.

"It absolutely scares me to death when I see the amount of residential growth coming on," said June M. Bachtell, Loudoun's director of economic development and chief sales person. "Then I think of the amount of nonresidential growth we're going to need to offset that."

A reckoning of the long-term costs and benefits of the new developments to the county is bound to be inconclusive. But a few rough forecasts are possible.

TRANSPORTATION. Projections show that by 2010 Loudoun will have twice as many residents and three times as many jobs as today. That guarantees one major problem: traffic. The boom will put a massive strain on the county's roads, even if developers help shoulder the traffic load with road improvements near their projects, planners say.

Rte. 28 today is a two-lane road that snakes through miles of open pastures. County officials say it will have to be transformed into a limited-access highway with five interchanges between Dulles and Rte. 7 if it is to handle the traffic generated by planned development, including the state's high-tech center near Dulles. Rush-hour traffic on Rte. 28 is already so bad, planning director Carr said, "It should have been four-laned yesterday."

And some county officials predict a shift in commuting patterns along Rte. 7, with the Xerox project putting westbound cars on the road in the morning for the first time. The existing morning eastbound traffic along Rte. 7 also may get a lot worse; one development proposal, Ashburn Village, calls for building homes for more than 13,600 people just south of Rte. 7 near Leesburg.

In addition to widening the major transportation arteries already in place, planners say, Loudoun will need major thoroughfares where roads barely exist today.

"See this," said Carr, pointing at a thin black line labeled Rte. 643 on a map. The line squiggled southeast from Leesburg toward Dulles, dead-ending near a stream. "That's a country dirt road. What we want to do is pave it, four-lane it and link it up with the Dulles Toll Road. It's a major east-west roadway to relieve pressure on Rte. 7."

Carr's can-do optimism may run into snags later. Who will pay for Rte. 643's upgrading -- and other needed county road improvements -- is another question, and one for which Carr has no ready answer.

While Fairfax County emerged as a big winner in securing road funds from the state at last winter's Virginia General Assembly session, Loudoun, lacking political clout, gained nothing.

The result is some derisive talk about the state legislature, and an expectation that developers will cough up significant amounts of money to improve roads on and around their projects. Last year alone, nearly $12 million in road improvements were offered by developers seeking county approval for their projects.

"They've got two of the shining stars of advanced technology straddling Rte. 28 -- Dulles and the CIT," said Supervisor Andrew R. Bird III, who represents a district in eastern Loudoun. "But then there's the black hole of the Commonwealth of Virginia and its inability to fund improvements on that highway."

ECONOMY. Although the inner suburbs in the metropolitan area have become increasingly self-sufficient in economic terms, outer suburban counties such as Loudoun have remained largely bedroom communities. Thousands of Loudoun residents commute to jobs in Fairfax County and the District; the county has three times more residents than jobs.

The development boom under way would alter that situation dramatically, making Loudoun more like its wealthier neighbors. The Xerox development alone, using conservative estimates, would double employment in the county, now just over 21,000. A handful of other projects likely would generate more than 1,000 jobs each. By 2010, Loudoun is projected to add 40,000 jobs; the county would have nearly half as many jobs as residents, according to the Metropolitan Washington Council of Governments.

All of those workers driving to jobs in the county, of course, means a change in life style in Loudoun, and one that many residents -- especially in the rural west of the county -- dread.

"Getting away from congestion is why a lot of people have moved here," said Supervisor Betty Tatum, who represents a district in the eastern part of the county. "But you have to have commercial growth to balance residential growth. Gone are the days when we could just lock the gates and say that's it."

The tremendous growth in eastern Loudoun also will strain county services such as fire and police protection, schools and recreation.

The current county plan calls for building four new schools, three in the east, over the next five years. The schools, to be financed by bond issues, are likely to contribute to rising tax rates as the county pays off its bigger and bigger debts.

In County Administrator Philip A. Bolen's proposed fiscal 1986 budget, which calls for a 9-cent rise in the real estate tax rate to $1.19 per $100 of assessed value, the two steepest increases are for schools and police. Those priorities are likely to be with Loudoun for a while.

LIFE STYLE. County planners say the western county hunt country will gain in population but remain essentially unchanged.

What will the eastern Loudoun boom area look like in 25 years?

"I would like to think we're going to look quite manicured and quite nice," said county land development chief Howard-O'Brien. "Certainly that's what we're striving for."

County officials say stringent zoning laws in Loudoun preclude the kind of commercial strip development that characterizes areas such as Rte. 1 and Rte. 7 in Fairfax by requiring development in clusters with plenty of greenery in between. They say the county can maintain its wide-open appearance.

Planners say land ownership patterns in eastern Loudoun, where a number of developers control large tracts of land, will work to the county's advantage by making planning easier. Said Bird: "We don't want patchwork quilt development. It's the large-tract developers who can keep the quality of life we want."

Over the next 25 years, eastern Loudoun residents will be more likely to work in the county, sit in traffic jams on their way to work and back, pay higher taxes and consider themselves more subject to the problems and opportunities of the Washington metropolitan area. In short, life styles in eastern Loudoun may become more like those in Fairfax or Montgomery or Arlington counties