Prince George's County finally is getting its share of Washington's Capital Beltway building boom, but there are warning signs that developers in the jurisdiction might be having more trouble filling its buildings than developers in Montgomery, Fairfax and Arlington counties.

In 1985, a total of 4.1 million square feet of new commercial space was built in Prince George's County -- stores, hotels, offices and research and distribution centers. The figure was nearly double the 2.2 million square feet built in 1984, according to a recent report by the Metropolitan Washington Council of Governments.

During the same period, however, the county had an overall vacancy rate of 16 percent, at least 900,000 square feet of unleased space, according to Thomas G. Owens, vice president of Spaulding and Slye, a commercial real estate brokerage and development firm.

Other leasing firms say the total vacant space in Prince George's is nearer the 1.2 million-square-foot mark, or nearly a quarter of what was built in 1985.

Owens said Prince George's 16 percent vacancy rate is "not a healthy figure at all," adding that in the metropolitan area only Arlington and Prince George's counties failed to realize a net gain in the total amount of space leased last year over that leased in 1984. These two counties were also the only ones in the region to build more in 1985 than in 1984, according to the COG report.

"If the COG report is accurate, then the county did experience unprecedented construction last year, and that's good for the local economy," Owens said. "But we have reached the point in the metro area where we have more unoccupied space than ever before. We're seeing a lot of 'see-throughs' empty buildings . And more in Prince George's than most places."

Prince George's developers had a net 515,000-square-foot gain in leased space in all buildings in 1984, but only a 396,000-square-foot gain last year.

In contrast, Montgomery County -- which, like Prince George's, also had a 16 percent vacancy rate for 1985 -- had a net 1.4 million-square-foot gain last year, up from 1 million in 1984, Owens said. Arlington County has a 4 percent vacancy rate, he said.

Fairfax County had a net gain of 4.4 million square feet in 1985 compared with 3.2 million in 1984, according to Spaulding and Slye's year-end figures. Fairfax has a 14 percent office vacancy rate.

"The figures may not seem terribly significant now for Prince George's, but if the sluggish leasing pattern continues there for another year or so, it could become a problem," Owens said.

But Battista Orcino, a broker with Julien Studley Inc., is less cautionary in his appraisal of the leasing market in Prince George's. "Prince George's County is alive and thriving and will do well over the next several years," he said.

Orcino compares Prince George's to Bethesda, saying, "There's so much construction going on, and the activity in the market far outweighs what is already built."

Prince George's economic development officials agreed and said they have no plans to moderate their aggressive campaign to woo developers. Plenty of cheap land remains at key intersections along the Beltway, and congestion is far less than in Northern Virginia, along Montgomery County's Interstate 270 corridor or in Bethesda, county officials said.

"We have 40 percent of the Beltway in Prince George's and 1,000 acres of land within it that is still undeveloped," said Jim Myrtle, development specialist with the County Economic Development Corp.

In addition, County Executive Parris Glendening has proposed a referendum that, if approved, would authorize $100 million in road improvements, Myrtle said.

"Many developers say they want to be within the sphere of Washington, but not trapped by it. From here, you can get to Baltimore or New York, Washington or Montgomery with less hassle than other places and use car, Metrorail or Amtrak," he said.

Orcino said that "to a certain extent we see the same type of tenants throughout the Washington area, but in Prince George's, the defense and R&D research and development people are flocking together. . . . We see a plethora of distribution centers for manufactured goods: Safeway, Giant, printers, computer warehouses, etc., because of the truck routes . . . . 18-wheelers need quick access to their warehouses."

Orcino said those desirous of a "very prestigious" image probably still will opt for Tysons Corner or Bethesda, "but the people out here are government contractors, and they must account for their rents."

He said lease-up time is longer in a county like Prince George's and tenants tend to expand or contract the size of their offices in tune with the federal awards they get.

Frank Mondell, senior vice president of Danac Real Estate Investment Corp., said, "I don't see the glut in P.G. like we see in Montgomery around Shady Grove at the Gaithersburg intersection . It's not here yet."

Danac moved into the Prince George's market in 1985 after building 14 two-story research facilities in Montgomery and is now finishing the second of three buildings totaling 170,000 square feet on an eight-acre site near George Palmer Highway in Lanham.

The first building was leased by Earth Observation Satellite Co. (EOSAT) after the government contractor of infra-red photographs looked at 137 different sites, according to Richard L. Stefan, development specialist with the Prince George's County Economic Development Corp.

Mondell said the county has made great strides in "improving its image. It used to be perceived almost totally as a blue-collar community. There was little or no good retail and the residential base made it difficult to build good office space. You saw a lot of B-grade buildings."

Howell Posner, director of marketing and acquisitions for the development firm of Birtcher Butcher, agreed. "In the past, P.G. has been criticized for not having enough executive-style housing, so big corporations moved elsewhere. . . . It's not the county it was 10 years ago."

Birtcher Butcher is developing the 30-acre Forbes Center in Landover, with its first phase including six single-story garden office buildings ranging from 18,000 to 35,000 square feet. Two are completely leased and the other four have "serious prospects," he said.

Many peg the county's evolving image to the decision in 1975 by Robert G. Depew & Associates Inc. to develop the 250-acre Washington Business Park along George Palmer Highway at Rte. 50. About 100 acres remain to be developed on the tract, which eventually will have about 30 research and distribution buildings.

Of the 1.7 million square feet in existing space, all but 50,000 of it is leased, said John Fernstrom, vice president of Depew. Major tenants include Sony, General Electric, ITT, Sperry-Rand Corp., Federal Express, Allstate Insurance Co. and the regional warehouse for Garfinckel's department stores.

"The Garfinckel's warehouse doesn't look like one," Stefan said. Landscape berms and an attractive brick front screen 18 rear truck docking bays from passers-by.

Fernstrom said "availability of zoned land and sewer" was a big factor in Depew's move to Prince George's. "We've attempted a couple of developments in Montgomery and been thwarted."

McCormick Properties Inc., a subsidiary of the Baltimore-based spice company, is developing the 230-acre Inglewood Business Park in Landover, with three five-story office buildings already on the site.

One of these, the 115,000-square-foot Inglewood Office Center Two, was finished in December, but it remains 100 percent vacant. "We do have prospects," said Robert Byrne, marketing manager for McCormick.

Byrne said a lot of the 1985 construction in Prince George's may be the result of developers "who were uneasy and uncertain about the Reagan tax plan and who wanted to get projects in the ground."

Prince George's, like the entire Washington area, is benefiting from the ready availability of construction money.

"Developers are like addicts," Posner said. "As long as lenders out there are pushing the money, the developers will keep investing it, so they'll have something to show when times get bad."