The population of Prince William County?

Joyce Eaby, public information officer for the county, whirled around in her chair to reach for the exact number in her files.

But she found neither files nor file cabinet -- just a lone potted plant sitting on an empty table.

Eaby threw up her hands. "Everything's packed!"

It was moving day for the county government in Manassas. At least six county offices were packing boxes of office materials to be taken out to the new $5.1 million James J. McCoart Administration Building 11 miles away on Davis Ford Road.

The booming county population that has nearly quadrupled in the past 25 years to 171,600 has created a corresponding county bureaucracy of 1,300 employes, outgrowing a maze of small government buildings spread out over four blocks at Lee Avenue and Peabody Street in downtown Manassas.

"It takes us from a bunch of ragtag offices to a 20th century building," said Prince William County Board of Supervisors Chairman G. Richard Pfitzner.

The first 350 employes were relocated during the past two weekends into 72,000 square feet of modern offices of the red brick McCoart building.

"This has really been complicated. There's a committee to oversee the move. Professional movers came in. Everything was tagged," Eaby said. "I've never seen a move quite like this."

County residents may have had a hard time getting through to the offices that had been opened for half-days last week, but residents from the densely populated eastern end of the county, including those from Lake Ridge, Dale City and Woodbridge, can look forward to conducting their business with the county government closer to home and under one roof, county officials said.

"The people that I've heard from are grateful they won't have to go all the way out to Manassas. This is the first time we'll be under one roof since 1950," said County Executive Robert S. Noe Jr.

Among the offices included in the move were the county executive, the chambers of the Board of Supervisors, the personnel office, the department of development administration, budget, planning, finance, the county attorney, the office of management information and audit, and a cashier's office formerly in Garfield. (A cashier's office will remain in Manassas.) In addition, the police chief, police communications and the fire and rescue administration will have an office at the McCoart building, while police and fire operations will remain in Manassas.

A District firm that specializes in office moving logged part of the county's office accoutrements: 3,500 boxes and 2,400 pieces of furniture including file cabinets, desks, chairs, typewriters, copiers, bookcases, video display terminals and the mainframe computer that acts as central command for each department's computer system.

Several reasons prompted the county government's decision four years ago to move eastward. Rents for buildings used in Manassas were totaling nearly half a million dollars yearly, said Pfitzner. Civic leaders had been talking about moving the county government into the county since 1975 when Manassas residents voted to incorporate into a city.

"We have the desire to have the county offices in the county," Pfitzner said. "Rents were running between $400,000 and $500,000 a year for space that was inadequate. We can't keep throwing money down the rat hole," he added.

Pfitzner estimated that rents for offices remaining in Manassas, which include the Department of Social Services and the mental health program, should run about $300,000 a year.

Some of the county-owned offices in Manassas were not exactly in the best shape, either. The fire marshal had condemned the white house that used to house Prince William's Circuit Court. The county jail built in the 1920s will also be torn down, Pfitzner said.

The Circuit Court, the magistrate's office, the General District Court and the Juvenile and Domestic Relations Court were moved into the county's new Judicial Center that opened in Manassas in the summer of 1984.

For Prince William County, one of the fastest growing jurisdictions in the metropolitan area, the James J. McCoart building is only the latest sign of the county's prosperity. Potomac Mills, a multimillion-dollar discount mall, opened two weeks ago and already has traffic backed up on Interstate 95 near its location north of the Dale City interchange.

In addition, the county plans to add to the Prince William Governmental and Recreational Complex on Davis Ford Road in the next two years by building an emergency operations center for the public safety department as well as the headquarters for the county water and sewer authority.

But right now, county employes are looking forward to the bright new offices with modular office furniture in coordinated pastel shades.

Suzanne Perez, chief of data processing in the Office of Management Information and Audit, said she is excited that the day has come when she does not have to worry that leaking floors will ruin the county's Hewlett-Packard computer system. The system plays a part in everything from emergency dispatching for police and firefighters to getting out the county payroll.

"It's worked really smoothly, much better than I thought," said Perez about the county's move.