A Northern Virginia developer yesterday made Loudoun County an unusual offer: Approve our request to build 15,000 houses and condominiums near Dulles International Airport, and we'll charge the new homeowners a separate tax to pay for needed roads, plus we'll build the county six schools.

The proposal submitted by Greenvest is one of what planners expect will be at least a dozen similar requests by today's filing deadline for such growth-plan changes. If approved, these proposals would dramatically alter the face of Loudoun, already the nation's fastest-growing county.

Backers said the proposals represent an ambitious attempt to overhaul Loudoun's land-use rules and transform the county into a model of faster growth that pays its own way instead of relying on the county budget to cover needed schools and other services. The new proposals are more creative, affordable and taxpayer-friendly, proponents say. Detractors say the plans are part of a risky effort to replace carefully considered building limits with costly, turbocharged development that will compromise the county's quality of life.

Another business group is proposing to take over more than 400 acres of county-owned parkland to build about 4,000 homes and a "main street" as part of a 1,550-acre project south of Leesburg. In exchange, the county would get a 250-acre park and $200 million worth of new roads, according to the proposal, put together by the firm behind plans for a baseball-themed town center development near Dulles.

Slow-growth advocates on the previous county board cut the number of homes that can be built in Loudoun by 80,000. Building and real estate interests, led by Greenvest, underwrote the campaigns of candidates who vowed to roll back those restrictions. A new board, more sympathetic to development, took power in January.

"The previous board would never have looked at this or talked to us about this," said Greenvest chief executive Jim Duszynski, adding that the former board had wrongly sought to repress much-needed home building. "This is a reaction, a market reaction to that."

Duszynski said his company is seeking to address long-standing criticisms that home building was spurring the demand for costly government services, such as schools, and that Loudoun's taxpayers were being asked to carry too much of the burden.

"Our proposal has residential growth that pays for itself, so they are going to have to shoot at a different target," Duszynski said.

Supervisor Mick Staton Jr. (R-Sugarland Run), who was elected in November, said he supports rethinking the county's development policies, especially in light of projects such as the one being offered by Greenvest.

"Loudoun County is growing. Loudoun County is going to continue to grow," Staton said. "The biggest complaint I always heard in the last four years is that growth isn't paying its fair share. If projects are going to pay their fair share, then it's something that probably deserves a look."

Staton said the county policies that restrict growth in eastern Loudoun's so-called transition zone, which runs from Leesburg to south of Dulles, should be reevaluated with an eye to his existing suburban constituents, who do not want more homes put in their neighborhoods.

"I don't support drawing an imaginary line across the county and saying we are not going to allow any sort of suburban densities across that line," Staton said.

But Supervisor James Burton (I-Blue Ridge), an architect of the county's efforts to slow growth, said the new proposals miss their mark on both fiscal and planning fronts. Burton noted that county taxpayers must pay not only for the construction of public facilities, but also for operating them in perpetuity.

Developers submitting proposals to change the county's plans are not taking a broad view of how the combined projects would affect the county as a whole, he argued.

"This is a backdoor approach to changing the county's direction. The development community knows that, and that's why they are submitting individual packages," Burton said.

"Why do we have to plop in 20,000 more houses?" he asked. "Are we obliged to accommodate every developer's plan? I don't think so."

Greenvest and other developers are taking advantage of a previously little-used opportunity -- with a deadline of today -- that allows landowners to propose changes to the county's planning blueprint for their property.

The Greenvest proposal would be the biggest residential development in Loudoun history. It would consist of four communities on about 4,200 acres. Included are plans for an age-restricted community, a hospital and a mix of houses, townhouses and condominiums.

Nearly $460 million in road improvements and school construction costs would be financed using community development authorities, a financing tool created by Virginia lawmakers that has been used in other communities, primarily for road building.

But schools, sewer lines and other infrastructure can be funded the same way, as Greenvest proposes. Under its plan, homeowners would not only pay their county property taxes, but they would also be assessed between $1,400 and $1,900 a year by the county on Greenvest's behalf. In addition, about $20,000 of the home price would go toward private construction of the six schools.

Loudoun school officials said they would analyze Greenvest's plan in the context of a system already facing booming growth.

"When you're pushing the pedal to the proverbial metal, is this going to be a win for the taxpayers?" asked the schools' top planner, Sam Adamo. "We'd have to really look at the total package."

The park plan was submitted yesterday, county officials said. Leonard S. "Hobie" Mitchel, who heads that project, is also president of Diamond Lake Associates, which will submit a plan for a baseball-themed town center development to the county today, he said.

Mitchel said his plan to swap the land for the voter-approved but as-yet unbuilt Philip Bolen Memorial Park would generate funds for major transportation improvements.

Greenvest gave $18,000 to winning Republican candidates in last November's election, more than any other contributor. "Smart business people don't just donate money for the heck of it," said Steve Calos, former director of Virginia Common Cause. "They realize it's a smart investment in their business to give money to candidates for office." Duszynski said he hopes only that his project will be judged on its merits.