Loudoun planners said yesterday that if the rural, western portion of the county is fully developed according to current land use rules, 69,729 additional houses would be built--a number that is prompting supervisors to call for quick action to prevent such construction.

County planning staff members presented their analysis of residential construction at yesterday's Board of Supervisors meeting. It is the first time the county has calculated what "buildout" in the west would look like.

Supervisors had requested the report because of concern over the amount of development west of Leesburg in areas that county plans say should remain rural. But much of the land in western Loudoun is zoned to allow one house on every three acres. With the declining profitability of agriculture, an increasing number of farmers and landowners are selling to developers who are replacing fields with subdivisions.

According to the Planning Department, there are now 11,300 houses in western Loudoun. If the land is developed to its maximum potential, the total will be 81,029 houses. County planners said they had not estimated how long that would take. Last year, they said, about 500 houses were built in western Loudoun.

"We're going to have to look at ways to keep that from happening out there," said Supervisor Scott K. York (R-Sterling), who was elected this month to serve as board chairman beginning in January. "We just can't afford it."

York and others elected to the board say they are concerned about the cost of providing services and building roads and schools in rural areas. They say that doing so would come at a substantial cost to the county--and to current taxpayers.

In addition, several newly elected supervisors have raised environmental questions about development in western Loudoun. They say they are concerned about the availability of well water and the discharge of effluent from septic systems.

Supervisors said the report about how much construction could occur in western Loudoun gives new urgency to proposals already under consideration, including boosting Loudoun's rural economy and paying landowners who agree not to develop their property.

Some supervisors said that when the new board takes office in January, they also will look into changing the zoning designation on land in western Loudoun to allow for less construction.

Supervisor James G. Burton (I-Mercer), who was reelected this month, said the report will encourage the new board to look for things that can be done immediately to slow growth.

"It's a scary number," Burton said. "This next board has to sit back and decide what vision it has for the rural area and then make sure that vision comes true."

Burton said he is looking into more stringent requirements that could limit the number of drain fields for septic systems because of health concerns. He said such measures could reduce the number of houses that can be built.

In addition, Burton said he is looking into stricter regulations for installing wells. He said he is considering pressing for a measure that would increase the minimum water-flow requirements.

Because most parts of western Loudoun are not served by public sewer or water, getting permission to construct drain fields and wells is essential for people who want to build houses.

Steve Stockman, a former supervisor who is developing the 140-acre Laycock Farm outside Hamilton, said he does not think such proposals would have much of an impact on his plans. County plans call for development to be concentrated near towns, and he said that is just what he is trying to do.

"If you're trying to do a rational, planned development, there are people who are going to see the benefits of it," Stockman said.

Supervisor Eleanore C. Towe (D-Blue Ridge), who also was reelected, said at yesterday's meeting that she is increasingly concerned about the cost of schools in western Loudoun to taxpayers.

"We see it coming like an enormous wave," she said of the development.