Finding that nice close-in lot on which to build your special house in Northern Virginia is difficult, and may soon be impossible.

According to local homebuilders and real estate agents, only a prolonged, diligent search -- usually accompanied by a stroke of luck -- is likely to produce a decent site these days.

Most of the remaining lots suitable for construction of single-family homes in the close-in suburban neighborhoods sell by word of mouth and rarely ever get into the Northern Virginia Board of Realtors' computerized multiple listing services data bank.

"Very few ever get into the hands of a Realtor or show up in the computer," said Doug Umlaus of Coldwell Banker's North Arlington office. Agent Betty Farrell runs the computer every Monday morning for a printout for a builder she regularly works with "just in case something shows up."

The lack of building lots in the older, more stable and sometimes elegant neighborhoods of Falls Church, McLean, Alexandria, Arlington and Annandale isn't a great surprise. They are all inside the Beltway, where many people are willing to pay top dollar for the convenience of the location, agents said.

Samuel Finz, executive director of the Northern Virginia Builders Association, said the area inside the Beltway is "developed out. It is not a phenomenon. It is what you would expect."

More than a dozen agents said the best way "and often the only way to find a single-family lot is to go straight to the tax maps" in various jurisdictions to see what land is available. Agents, or buyers, often contact property owners directly to see if they are interested in selling an empty lot.

"It is a lot easier if a potential buyer knows the location he wants. It is easier to find out there is nothing available," said one agent.

Those that are available are often the leftovers, such as a single lot that a builder deemed "unbuildable" when a subdivision was built 20 years ago. There are often on steep slopes that make building more expensive, agents and land specialists agreed.

When asked how he deals with potential clients who show up with plans in their hands asking for help in finding a lot close in, builde Robert Young said he was a somewhat set reply: "I will try, but I don't promise success."

"If you could get a half-acre, you would have to pay $80,000 to $90,000 at least. Then you have got to multiply that by at least three to get your construction costs to add to the price of building," he said. That can bring the total cost up to $340,000 for a four-bedroom house.

When that happens, agents warned, the would-be home buyer has to realize that what he may build may very well overshadow the surrounding homes in the neighborhood, which could sell for anywhere between $100,000 to $200,000.

"Building close in is not something you should do unless you are building exactly what you want in the exact location where you plan to spend most of the rest of yourlife," warned one agent.

It is hard to build the same square footage on a single lot for the same price as a builder can produce the identical finished product in a large development, simply because of the economics of having construction workers "running here and yonder," explained one builder. "Subcontractors cost more, and it costs more to have a construction supervisor overseeing one house rather than 20," a Falls Church builder explained.

Builder Young recently bought up, one at a time, seven lots on Hunting Avenue near Tysons Corner where he built a series of Victorian farm houses. But he sold most of the homes before he started construction. The small lots were substandard according to current county codes, but the zoning on them existed long before the Fairfax County comprehensive land use plan was developed.

Cynthia Bartholomew, co-owner of Holley, Hargett and Spain's office in McLean, "ran the computer" this week searching for lots available for construction of detached houses inside the Beltway. Her search turned up just three in the general McLean area, ranging in price from $27,000 for an 8,857-square-foot lot in the Pimmit Hills area near Tysons to $100,000 for a 19,000-square-foot lot in Frankling Park near the Arlington line in Fairfax.

In North Arlington, 10 lots showed up, including one in the Lehigh area that could be subdivided. Two were listed in South Arlington. The most expensive Arlington lot was 25,000 square feet in Chain Bridge Heights for $150,000.

"There were only seven in Alexandria," Bartholomew said. Most of those were close to Seminary Road. In general, prices ranged from $60,000 to $125,000.

Two showed up in the Falls Church area. Both are smaller than 20,000 square feet and are priced below $30,000.

According to a recent survey of U.S. housing markets by the Lomas & Nettleton Co., single-family lots at approximately a quarter of an acre now cost between $25,000 and $30,000 five to eight miles outside the Beltway and $20,000 to $25,000 further out. Closer to the Beltway, the report found, lots cost about $50,000 for a third for an acre. No statistics were reported for lots within the Beltway.

As one agent explained, "There simply are not enough left to count."