The Prince William School Board has approved plans to build a $530,000 addition to an elementary school in the rapidly growing area around the city of Manassas.

The decision last week to add on to Coles Elementary School comes at a time when school officials are trying to find ways to provide additional classroom space in overcrowded schools and, thus, to limit busing.

It is a problem that Prince Williams County alone among Washington area jurisdictions is facing. While most school systems are experiencing enrollment declines that are expected to continue. Pince William is anticipating growth in its enrollment over the next few years.

To meet that growth, the county already has committed itself to building three schools plus additions at four others.

"Our enrollment increased dramatically, then remained steady for the past few years," school spokeswoman Kristy Larson said. "But we are expecting an increase again. The site plans [for proposed new residential communities] have all been approved. We're just waiting for the new influx of residents to move on in."

Prince William's population doubled over the past 10 years, giving it the largest increase in the metropolitan area. Projections call for even more growth in the next 10 years. So although the number of school-aged children is decreasing, the number of new students moving into Prince William is expected to make up for any decrease.

Even so, county planner Roger Snyder said growth in the county has not been uniform. Some county schools now stand half-emply because there are not enough students within their jurisdictions.

The board has tried to alleviate this problem by juggling school boundaries and busing students from heavily populated residential areas in the eastern part of the country to less populated schools in the more rural west. But parents and students alike have vehemently protested these changes.

Larson said the school board will continue to change boundries and bus students if necessary, but she said the proposed new schools and additions, when completed, will do much to eliminate the need for those unpopular steps.

She said the board plans to build the new schools in areas where the population is expected to increase and then let the community "grow into them."

For example, Lake Ridge Elementary School, scheduled to open next fall, is expected to have only 337 students, although its capacity will be 650 students, she said.

Funding for the new schools and additions is expected to come from various sources. The Coles Elementary addition, for example, is being funded through the sale of a county school to the Manassas Park school system.

About 100 Coles students attend class in five portable trailers next to the school because of overcrowding.

"Those youngsters have to come in and out of the building in bad weather to use the bathrooms, the library and cafeteria," Principal Robert Blevins said. "We've done all right, but as our population grows so must the school."

The board plans to fund a proposed $23 million, seventh high school, to be completed in 1989, with a county bond and state grants. Funding for the third new school -- an elementary school in the eastern part of the county -- will come from two state grants. Funding for the additions will come from state grants and the school's budgets.

Currently, 35,255 students attend the county's 49 schools, a 2 percent decrease from last year. But county planners project enrollments to start increasing again, reaching 35,844 in 1987, the last year projected.