When an old house with as little as 1.5 acres of land hits the market in Northern Virginia these days, it's not the first-time buyers or the old-house lovers who are the first to scramble for the right to buy the property.

It is the builders, a special breed of builders who are rapidly becoming known as "in-fill" specialists. These are developers who are looking for land in close-in locations that can provide hard-to-find lots for new home construction.

If the property happens to have three to five acres, the scramble for it is even more hectic.

Builders generally do not care about the age of the existing house or even its condition. What they want to know is if the land can be subdivided or if, by chance, the existing resident's home occupied more than one lot, perhaps even three or four.

Builders want to know if they can build around the existing house, but that is not their first concern. They want to know if there are problems subdividing to get the chance to build in-fill houses.

Later, they will decide if the existing house is valuable historically or structurally sound and can be made more attractive by rehabilitation or simple updating. In some cases, the builder already knows a first-time home buyer who is looking for a chance to buy a house that needs work.

Generally speaking, new homes inside the Beltway are rare, and builders who can grab the little pockets of land that become available are finding they can turn handsome profits on the products they build. However, they are not generally building the typical tract house. They say they are building "quality on smaller lots in prime convenient locations."

"The name of the game again is location, location, location," said one so-called in-filler builder.

Fairfax County Supervisor Nancy Falck, who represents the Dranesville district, said those in-fill parcels demonstrate some of the best usage of Fairfax County's cluster zoning category. Builders are able to deal with topography problems, save trees and preserve the important quality of existing neighborhoods while providing badly needed close-in housing at the same time, Falck explained.

Builder Harvey Borkin is ready to begin construction on three lots that have been cut out from an original parcel that includes a turn-of-the century home across the street from Longfellow Intermediate School in the Falls Church/McLean area. He is preserving the understated three-story Victorian farmhouse with its hardwood floors and formal entry.

But more important, he is saving giant hickory, walnut and pecan trees that are at least 100 years old. Borkin already has sold the three in-fill houses he plans to build even though he has not yet turned a spade of dirt. The houses sold for $250,000, at least $60,000 above the prevailing resale price in the immediate neighborhood.

"I like to build inside the Beltway. There is a market for these things," Borkin said. "I take pride in the trees we save." The walnut, pecan and hickory trees that provide an umbrella over the front yard of the old house he's building around are "very valuable," he said.

Like Borkin, other in-fill developers such as David Counts, president of Tipco Homes, and McLean developer Robert Young agree that the sales of potential in-fill building sites are often by word of mouth. They say the competition for the sites is strong, especially if the property is already subdivided.

That fact can cut 18 months off of site plan reviews and rezonings, explained one Fairfax land broker.

"What is left out there is the hard-to-build-on lots," said Young, who recently built a simplified Shaker-style house by adding on to an existing garage along Old Dominion Drive within walking distance of the middle of McLean. He sold off another in-fill lot in the same package to another in-fill specialist, Ken Foley, who built a gray Victorian. The two houses are now competing in the sales market.

Tipco's Counts has completed at least 10 in-fill projects in the Virginia and Maryland suburbs. Often, the original houses on the properties Counts buys do not have historic value. Many are typical 1950s ramblers that were built on large plots of ground. Sometimes it can take up to two years to get such a property engineered and rezoned before building can start, builders agreed.

"Sometimes we redo the existing houses ourselves. Sometimes we help the buyer do it, and sometimes we sell it outright," he explained.

Counts said it is important to make sure that whatever is done with the original structure is done to make it compatible architecturally, visually and, when possible, economically with what Tipco builds around it.

Counts said he is especially pleased with the final development of Franklin Cluster, off Powhatan Street in the Falls Church area near the Arlington line. The development is next to natural parkland. A few miles away, Tipco built another small development where Counts offered a large two-story model complete with a two-story family room and library at prices $20,000 to $40,000 below market for similar housing. Ironically, Fairfax County is now putting the finishing touches on a major park next door.

"The advantages come not from having the existing house but from the real good locations in established neighborhoods," Counts said.

Near Tysons Corner along the Idylwood Road corridor leading toward the Merrifield and Fairfax Hospital area, residents of some older neighborhoods that were bisected by the Beltway and I-66 have watched as construction of new Victorian homes in the high $100,000 to the low $200,000 range have attracted younger residents.

Built by Taylor/DePalma Co., the homes feature intricate gingerbread detail and interior features from the Victorian era. The neighborhood is a prime in-fill area, convenient to I-66 and Tysons Corner, inside the Beltway.

Monte West, whose company is called West Homes, is primarily a custom home builder. Right now, because of a hangup in another development, West Homes is building in-fill homes along Old Dominion Drive at the Beltway border.

The two-story colonial homes sell for approximately $400,000. Three are planned. One has sold.

"The lots are well located. McLean has a major name," West said this week. "I saw these lots just by driving by. Sometimes when you see land you have to keep going back to the owner. One year they think they are going to build on it, but the next they may decide to buy a beach house, and be willing to sell off the lots."

Developer Young, always on the lookout for close-in land, recently completed a Victorian development in which the details of the houses actually were copied from the original house on the property. Those houses sold for as high as $280,000 in a neighborhood where you can still buy a home for $120,000.

Borkin also is building expensive in-fill housing. He is involved in a Bethesda project at Bradley Boulevard and Fernwood Lane, is building two Cape Cods and a Tudor on wooded lots that will sell from $425,000 to $500,000, and is restoring the two older houses that were on the land when he bought it. The new ones will be compatible with the old, he said.

Developers in general said keeping the new compatible with the old is an extremely important factor. It is significant not only to guarantee the developer the best return on his investment but also to retain the good will of the community, said one agent.

"If you are in the in-fill housing business, botching up one development can really do you in," said one builder. "If you mess up, then the whole neighborhood is mad and will fight like hornets the next time you want to build anywhere near their houses." Samuel Finz, of the Northern Virginia Builders Association, said it is hard to distinguish between those builders who put up individual houses and those who are now being identified as in-fill builders.

Finz called the land inside the Beltway "almost urban," increasing the need for builders to develop those rare close-in parcels.