The Loudoun County Board of Supervisors, trying to limit residential development in the bucolic countryside north of Washington Dulles International Airport, has told developers they will be required to start paying fees to provide county facilities for the new communities.

In a precedent-setting move for a Virginia locality, the Loudoun supervisors this week approved the concept of a strict payment formula that details what level of development will be allowed in the Ashburn area north of Dulles, the county's most intense development spot at the moment.

The supervisors said developers will be allowed to build a maximum of four homes on each developable acre on the rolling farmlands and forests of the Ashburn area in eastern Loudoun. But developers only will be permitted to build to that density if they contribute enough toward construction of county schools, roads, parks, libraries, fire stations and to the county's rural preservation program.

This fee-schedule concept has raised concern among some county supervisors, who believe that developers would contribute more to the county if they were allowed to do so on a more voluntary basis.

The county has yet to devise exact payment schedules. But under one plan the county's staff has suggested, a builder developing an 800-acre tract at the maximum density of four units per acre would be required to pay the county $2,138 for each house. If the developer only wanted to build 2.4 homes an acre on that tract, the payment to the county would be $819 for each house.

It is uncertain whether conservative Virginia courts, often inclined to protect business and land development interests, will uphold the requirement. Developers have opposed the Loudoun payment plan and have questioned its legality.

Other subdivisions in eastern Loudoun, such as Sterling Park Countryside and Sugarland Run, range in density from 3.1 to 4.2 units per acre. But these developers did not earn their densities by satisfying a monetary formula.

The decision favoring the density formula came when the board voted 6 to 2 for a planning document, called the Dulles North Area Management Plan, that is meant to guide future development in the 34 square miles northwest of the airport.

At the moment, that area is a rural backwater with only about 1,000 residents, many of whom live in white-painted farmhouses amidst woods, corn and soybean fields. But county planners estimate that by the year 2020, the north Dulles district, with the 19th-century village of Ashburn at its heart, will have a population of 74,888 and will offer 28,800 jobs. Today, the entire population of Loudoun County is under 70,000.

Three major development plans already have been filed with the county for the north Dulles area. If approved, the developments would consume one-fourth of the area's open space and add about 11,000 homes -- half of what the plan suggests for that area.

Planners estimate that to cope with that level of development, the county would be forced to spend $221 million to build new schools and add police, fire and recreational facilities. If Loudoun residents were to bear the brunt of those costs, it would add 40 cents per $100 of assessed valuation to the real estate tax rate, county officials estimate. The current rate is $1.13.

Such a prospect prompted Loudoun County Board Chairman Frank Raflo to push for a formula detailing how much developers would have to contribute to the cost of growth in the county.

"All this payment plan attempts to do is tell the taxpayers of this county what the cost of growth will be and who will attempt to pay for it," Raflo said.

But developers have been adamant in their opposition. "It's wrong and it's illegal," said Samuel A. Finz, chief executive officer of the Northern Virginia Builders Association.

Finz said the payment system would undermine the commonly used system of proffers, endorsed by the Virginia General Assembly and state courts, under which builders negotiate voluntary contributions for public improvements with county planners and politicians on an application-by-application basis. Finz said he expects that the formula Loudoun has adopted will make developers less willing to contribute toward construction of county facilities.

Two of the three Republicans on the county board voiced similar fears.

"It doesn't take into account the different types of lands. . . it's ironclad," said supervisor Andrew R. Bird III. Supervisor Steve W. Stockman called the formula "foolish." Stockman and Bird cast the two dissenting votes on the Dulles north plan.

Despite these philosophical differences on how to encourage developers to pay for the growth they are promoting, the Loudoun board now must apply the formula to rezoning applications that have already been filed.

Alan I. Kay, a Bethesda-based developer, in a joint venture with James Flood of Associated Investments Inc., wants to rezone 1,580 acres immediately to the east of today's Ashburn Village for a planned community. The huge development would include 5,451 homes, 3.1 million square feet of office and industrial space and 200,000 square feet of retail space.

The developers thus far have proffered the county $19.5 million in exchange for an average density of 4.3 residential units per acre. Rosser Payne, the consultant for the project, said he is negotiating with the county over how the density is calculated to see whether the project exceeds the county's new standards. The board is due to vote on the application next month.

In addition, Fairfax developers William A. Hazel and John D. Stockley Jr. want to build 3,830 homes, 250,000 square feet of retail space and 75,000 square feet of offices on a 1,274-acre site known as Ashburn Farm, southwest of the existing village.

Xerox Corp. is seeking to build 1,800 homes on its 2,267-acre site, in addition to 20 corporate office centers, 200,000 square feet of retail space, two hotels and a conference center. The supervisors also hope to act on that proposal in November.

However the board votes on these plans, Ann B. Kavanagh, the supervisor who represents the Dulles north area, said the goal will be to retain some of Loudoun's rural flavor.

"It's a choice between an urban setting and a rural setting. We want to preserve some of our rural nature," she said.