Larry Horton, marketing director of Fralin and Waldron Inc., a Roanoke-based apartment-building firm, has spent two years searching for land in Fairfax County planned for multifamily housing. He's still looking.

"Where can I find the 10 to 20 acres I need for garden apartments at a reasonable price?" Horton asked. "It's not there."

Ronald G. Johnsey of Dimension Development Co. tells the same story. "It's almost like running into a stone wall trying to find apartment land in Fairfax County," said Johnsey, vice president of residential development for the Dallas-based firm.

So both companies have joined the exodus of development south over the Occoquan River to Prince William County to build apartments to serve the Washington metropolitan market.

Thousands of acres planned for multifamily use, and land costs 30 to 50 percent lower than in neighboring jurisdictions, make Prince William County the most attractive place today in the region for apartment construction, developers say.

The result is a boom in multifamily projects in Prince William. Last year, the county issued building permits for 880 multifamily units, up from 30 in 1983, while the office of planning has applications on file for another 5,097 units. Multifamily units accounted for 30.1 percent of the county's residential construction in 1984.

By contrast, only 19.7 percent of Fairfax County's residential permits in 1984 were for developments of more than 20 units per acre.

Developers say they would gladly build in Fairfax County, but it's hard to make the finances work because land prices and development costs are too high.

With only 0.19 percent of the county's 233,341 acres of developable land planned for multifamily use, apartment land in Fairfax County is at a premium.

Horton, for example, said he was offered land at Centennial, the mixed-use project at Fair Oaks, for $16,500 a unit. "You can imagine what my rents are going to be," he said. "Who's going to pay for that? They'd rather get a mortgage."

Instead, he headed for Prince William, where he is purchasing land for $4,000 to $6,000 a unit. He has four projects in the pipeline.

After a similar search in Fairfax County, Johnsey's company, Dimension, has two projects under construction in Prince William. The 372-unit complex beside the Manassas Mall on Route 234 should be completed by July 1; and another 190 units nearby on Coverstone Drive, by next February.

And in Prince William, developers have an abundance of land to choose from. The county's 1982 comprehensive plan shows 1,150 acres earmarked for multifamily use, a figure that has increased with subsequent rezonings, according to planner John Schofield.

In addition, developers say it is easier to work with Prince William County's government than with its more urban neighbors. "They are pro-development, especially when you compare them to Fairfax County and Montgomery County," said David Strauss, a partner in Houston-based MHB Cos.

In Prince William, he can find land already zoned and get site plan approval within one year, while in Montgomery and Fairfax he would have to budget three years to get through the rezoning and plan review process, Strauss said. Most developers cannot afford to tie up their money that long, he asserted.

MHB Cos. expects to break ground this fall on a 417-unit project on Route 1 in Woodbridge near the Potomac Mills factory outlet mall now under construction.

This boom in Prince William County has raised the specter of a glut of apartment units, however. Local zoning attorney Michael Lubeley said he doubts all the units now being planned will be built.

Developers, though, remain bullish. They say all the economic indicators point to rapid growth in Prince William and, with it, continuing demand for rental housing or condominium homes.

Its population is the fastest growing in the metropolitan area, according to the Washington Metropolitan Council of Governments. Conservative figures from the U.S. Census Bureau show the county's population growing from the 144,703 recorded in 1980 to 220,000 by the end of this decade.

The job base is growing, too. The Virginia Employment Commission estimates that the number of people working in the county rose by 32 percent from 1980 through 1984. Now 25,580 of the county's work force are employed in Prince William, up from 19,272 in 1980, the commission says. Like the rest of Northern Virginia, Prince William also enjoys a minimal unemployment rate that hovers between 2 and 3 percent. Wages also are rising, and developers say they are gambling on rents continuing to increase by between 7 and 15 percent annually.

Strauss also said that, unlike Texas, which is flooded with apartments, Northern Virginia has a planning process that will deter excessive multifamily construction: "It takes you so doggone long to get it zoned, they'll never be too many there."

Even if the tax-free financing now available for apartment construction dries up with proposed changes in the tax laws, Strauss doubts that construction will halt. With interest rates now dropping below 12 percent, conventional bank loans again can underwrite multifamily building, he said. In 1984, six of the nine apartment projects that sought site plan approval in Prince William County applied for tax-exempt bonds.

Another factor enticing developers to Prince William County is the steady march of jobs beyond the Capital Beltway. The growth of employment centers at Tysons Corner, Fair Oaks and along the Dulles corridor stimulates the housing markets further west that are served by good highways, developers say.

"We and others have targeted Manassas as one of the growth areas of Northern Virginia," said Wallace F. Holladay, a partner in Holladay Corp. The Washington, D.C.-based company has two applications on file for apartments in and around Manassas totaling 2,330 units.

Thomas F. Bozzuto, regional partner of Oxford Development Corp., the third-largest apartment building company in the United States, credits the opening of I-66 inside the Beltway plus International Business Machines Corp.'s plans to expand its Manassas plant by 1,000 employes for the surge of interest in Manassas. Oxford has options on two apartment applications in the Manassas area and also is constructing units in Lake Ridge, along the I-95 corridor.

Locally, the apartment boom has brought mixed reactions. Prince William County Supervisor Guy Anthony Guiffre, who represents the rural western parts of the county, said he's anxious to see that multifamily housing does not skew the market. County planning director Roger Snyder is preparing a report for the supervisors on the impact of apartment construction on county resources.

County Board Chairman G. Richard Pfitzner welcomes the apartment activity, however. "We've had a dearth of them before," Pfitzner said. "When youngsters get married here, they have to move up to Falls Church."

Unlike town houses, apartments put less pressure on the county budget because they usually are occupied by singles, newlyweds or the elderly, Pfitzner said. Each unit also generates fewer car trips daily, he added.

As for fears raised by citizens at public hearings that apartments will attract low-income people throughout Northern Virginia, Pfitzner calls this a "misconception." With rents starting between $400 and $500 for one-bedroom units, they cannot meet the needs of the poor, he said.

Citizens also are protesting the traffic that apartments will bring to already-clogged roads. About 1,000 names were collected for a petition protesting the Pomeroy Development of Fairfax's application for a 648-unit, low-rise complex on the banks of the Bull Run outside Manassas that the county board approved in May. Residents said traffic already crawls along Route 28 from Manassas to I-66 in rush hour and the apartments would worsen the situation.

A local opponent of apartments, Fred J. Thiesing, also is collecting funds to file suit against the Pomeroy application for 1,400 units on Liberia Avenue outside Manassas. That application has not yet come before the county board of supervisors.

Meanwhile in Fairfax County, planners are surveying land to see what parcels might be planned for apartment construction. Until more are made available at a reasonable price, developers say Prince William will get their business.