Supervisor Edgar S. Wilbourn III launched his most recent attack on Prince William County's slow-growth plan yesterday, saying a discrepancy between the number of available residential lots could cost the county hundreds of millions of dollars and increase the projected population by at least 100,000.

Such a population boom also would mean a greater need for schools and roads, which would "cost us much more money than the original plan detailed," Wilbourn said at a news conference in his offices yesterday afternoon.

The Gainesville Republican has actively opposed the Comprehensive Plan's "Rural Crescent" initiative, which sets aside part of the county as a rural enclave, since its proposal.

Wilbourn said yesterday that if the Comprehensive Plan's population projections are in error, they would affect budget considerations based on the earlier numbers.

When the Comprehensive Plan, the county's blueprint for land use, was proposed in December 1997, the number of preapproved housing units was 43,000. But in a staff report submitted to the Board of County Supervisors last month, the number was listed at almost 66,000 units. Wilbourn contents that number significantly affects, and essentially rearranges, the agreed-upon plan for controlled growth.

"The Comprehensive Plan is a major growth-management tool and a financial management tool" for the county, Wilbourn said, adding that a "huge confidence gap" now exists between him and County Executive Bern Ewert, who developed and proposed the plan almost two years ago.

Ewert was unavailable for comment yesterday.

Kathy Bentz, county spokeswoman, said that Wilbourn's concerns were addressed at last week's board meeting, and that Ewert's staff "plans to find out why exactly there is a difference to make sure it's right."

"At the board's request, a report about the build-out information, including vacant land, will be detailed," Bentz said. "But I can't say why one number is different than the other. What the staff is saying is that we'll take a look and come back to the board with what we find."

Nimet El-Alaily, the county's deputy planning director, said the difference of 20,000 units sounds right, as many revisions have been made to the plan since its inception and approval, in August 1998.

"I'm very comfortable with my reasons as to why the two numbers are different," she said, "because logically, they would be different."

El-Alaily said that members of her office approached supervisors each time the numbers changed and that she plans to "show again why they changed."

Factors that ultimately led to the difference include a reduction in the initially proposed Rural Crescent area and an affirmation of zoned land, El-Alaily said. "We had to go in and include those who had been zoned and those who were going to convert agricultural land to units," she said, adding that several months passed between the inception and the approval of the plan and that such changes inevitably occur over time.

The Comprehensive Plan--with its controversial "Rural Crescent" of agricultural land--is designed to cut more than 27,000 housing units. It also sets aside about 80,000 acres as a rural belt stretching from the Quantico Marine Corps Base to the Loudoun County line, reserving the area for farms and subdivisions with single-family home lots that are 10 acres or larger.

The plan also includes new guidelines for "proffers," asking that developers pay as much as $15,668 per house toward the cost of schools, parks, libraries and fire service.

Prince William residents have embraced the plan, contending that such an initiative will slow the growth of the otherwise rapidly expanding county.

Wilbourn, who has been outspoken in the past about his desire to see the county continue its economic expansion, said yesterday that the estimated additional 100,000 people that would be added to the projected population is "just too much."

"I'd like see [a projected population of] about 420,000," he said, "and something close to 500,000 is too much growth. I'm not in favor of that much growth, and that's why we need to find the right number and use that to fix the overall plan."