The Prince William County school system has 389 students more than expected at this point in the school year, a product of the robust economy and brisk home sales.

Not only is there more housing being built in the county but there has also been a noticeable increase in the number of single-family houses, as opposed to town houses or apartment units, according to the county's Planning Office.

The school system estimates that each single-family house produces one school-age child. The number of children living in town houses or apartments is far lower. The school population stood at 53,049 as of Sept. 30, compared with the estimate of 52,660. The most growth was in the number of first-graders and 10th-graders, with 218 and 444 more than projected, respectively. Some grades had fewer students than expected.

A difference of 389 students is only about 1 percent more than what the school system planned for, but Superintendent Edward L. Kelly said those extra children make a difference.

"You'll get more state aid, but your state aid never ever covers the full costs of educating that child," Kelly said. "It does create some problems."

The county has a reserve fund to pay for the 389 students, but that fund is also used to provide extra money to schools that have continued to grow through the school year.

"This is just a bigger change than we would normally expect," said schools Finance Director David Cline. "That makes it more difficult as you go through the year."

County Planning Director Rick Lawson said Prince William usually generates 2,400 to 2,500 building permits a year. But in the last two years, that rate has been closer to 2,800 to 2,900 a year.

The school system based its estimates on those figures, only to see the building permit rate rise again. As of August, 3,274 permits had been issued over the preceding 12 months.

"This has been a really steady, sustained type of growth," Lawson said.