A flurry of proposals for new development delivered to Loudoun County planners calls for adding tens of thousands of houses to existing county building plans, on sites from the rural west to land near Dulles International Airport.

By yesterday's submission deadline, the county had received 21 such proposals -- an unprecedented rush that indicated a coordinated effort to roll back Loudoun's strict growth controls.

The plans came in under a seldom-used process that allows landowners to seek changes in the countywide development blueprint if circumstances have changed significantly or if they think they have a creative idea that would augment the county's land-use vision.

"We've never had a situation like this," said planning director Julie Pastor. "That suggests there's something bigger at issue."

That something is a debate about how big America's fastest-growing county should become, and how it should get there.

Builders already have permission to boost the number of houses in Loudoun -- about 83,000 -- by nearly 50 percent. County officials say current plans, even with building limits put in place by a previous board last year, allow tens of thousands of additional houses as well.

The proposals submitted by yesterday significantly raised that number. Home builders and other advocates say county plans must change to accommodate the diverse pay scales of the county's growing employment base, and they argue that suburban lots are becoming scarce.

Those advocates also say land-use circumstances have changed.

Early this year, county supervisors voted to allow public water and sewer lines crucial for development to be stretched throughout a 23,000-acre swath of Loudoun that sits between the county's booming, suburban east and its more rural west. Known as the "transition zone," the area had been sparsely developed because of a lack of utilities. But recently, there has been increasing pressure from builders who eye its proximity to the airport and nearby businesses.

Developers have offered what they call a series of creative -- opponents say audacious -- approaches for financing large new communities there and elsewhere in the county, including private taxing districts to pay for roads, commitments to build schools themselves and an offer to swap road improvements worth millions of dollars for county-owned parkland.

Rick Entsminger, an executive at Elm Street Development and vice president of the Loudoun chapter of the Northern Virginia Building Industry Association, said builders are pushing for policy changes now after years of difficulties with the previous board, which cut the number of houses allowed in Loudoun by 80,000.

"They feel like they have an opportunity to get a fair hearing on their proposals," Entsminger said, adding that some hope to overturn county policies blocking suburban development in the transition zone, which stretches from Leesburg to south of the airport. "There are people who feel . . . here's an opportunity to challenge that," he said.

Some in the county, however, say the burst of proposals has raised a series of difficult questions that demand impartial analysis.

Susan Klimek Buckley lives in the Sterling area and founded the Eastern Loudoun Civic Network to address concerns about schools funding and development, among other issues. She says claims in the latest development proposals, such as the promise that many of the residential projects will pay their own way, must be carefully scrutinized.

"What is the ultimate cost to the taxpayer?" asked Klimek Buckley. "How does it affect the quality of life? What does it mean when you have all these people moving into the county? What about the parks and traffic congestion."

"I don't think you can take what they say at face value," she added. "Our leaders, the Board of Supervisors, have a duty to do more than that."

The proposals submitted to the county range in size and approach.

Greenvest L.C. proposed a 15,000-unit project near Dulles that would be the largest in county history. Pennsylvania-based home builder Toll Bros. has submitted plans for a planned community next to Greenvest's project. The Peterson Cos. proposed a mixed-use development south of Leesburg.

Many of the plans would sharply increase the number of houses allowed per acre. A 420-acre development called Ferncliff, north of Leesburg, is zoned for 41 houses. The applicant is asking for 660 as part of an age-restricted community.

Another development, outside the tiny town of Hamilton, could have six houses on 67 acres under current rules. The applicant wants to build 86.

Northern Virginia developer Randolph D. Rouse proposes putting more than 3,000 houses on his 927 acres near the airport. Rules generally allow one house per one to three acres.

Diamond Lake Associates submitted scaled-back plans for a baseball-themed town center to include a hoped-for major league ballpark. It submitted proposals for two separate sites, one 39 acres, the other 47, saying it hoped this would be the first phase of a much larger project. Diamond Lake asked for the ability to build high-rise developments on those properties.