What do you do, short of selling the place and moving, when you live in a two-story row house and need more room? Thousands of Washingtonians are caught these days in the bind of wanting to stay where they are for a variety of reasons (not the least of which is high mortgage interest rates). Row house dwellers often find theirs is a double bind -- besides cost pressures, zoning restrictions block them from building up or adding on to their houses.

Builder Dan Lowenthal has made a 30-year career out of solving such problems -- he simply digs additions under houses, converting cellars into remarkable living spaces.

Excavating under an existing house, particularly an old house, is not easy. It's a lot like coal mining: you have your mechanical digger and your pick and shovel -- and the danger of a cave-in. If you don't run into rock or water, the archeological side of such work makes life interesting. "I keep expecting to run into a dead body," says Lowenthal, tongue-in-cheek, "but usually all we find are old bottles -- liniments, patent medicines, that kind of thing."

Lowenthal is always seeking new ways to solve the problem of digging people out. Just now he's looking for what he calls a "laxative for rocks," a substance that will break up rocks without the dangers of blasting.

Another downstairs aficionado is Capitol Hill architect William Ward Bucher, who has helped design more than 30 different basement additions, including his own, that required some digging out.

"Digging out a basement is tricky business," says Bucher. "For example, most people aren't aware that below the foundation they see in their basements are footings, that is, broad piers that support the house. In digging down, if you don't have skilled people, they'll just knock off the side of a footing or remove it -- then you have real problems." A previous owner of Bucher's house, for example, knocked off the side of a footing; as a result, the back wall has a dramatically wavy look created by an instability Bucher is now trying to correct.

To reinforce weak or inadequate footings, both Lowenthal and Bucher recommend strong underpinning -- steel reinforcements and concrete that will shore up a building. If you don't know what you're doing, you could end up with a pile of rubble where your house should have been.

Water is undoubtedly the biggest problem in any basement. Lowenthal employs an elaborate technique of keeping water out. He installs a graveled drain around the interior of the footings that leads to a sump pump pit, if there is even the slightest sign that water is going to become a problem.

When you are digging a space that is deeper than its neighbors in a block of row houses, you're bound to run into water. Recognizing water's insidious nature, Lowenthal puts what he calls "bleeds" into the wall, providing the water with paths not harmful to the completed structure. When the contractor begins to build up the floor, he starts with a four-inch layer of gravel, covered with a polyethelene vapor barrier on top of which he pours a four-inch concrete floor. He may also put welded wire mesh into the floor for added strength.

Bucher recommends installing a floor drain to avoid a problem he encountered when his boiler burst: a layer of rusty water all over the basement floor.

A common use of a newly excavated basement, according to Bucher, is as a rental unit that will help carry the cost of rehabbing the upstairs. In one such basement, which already had the legal ceiling height (7 feet 2 inches), Bucher helped the owners excavate the living room area of the apartment to an eight-foot height.

"Just the difference of less than a foot made the stepdown living room seem as if it were 10 feet tall," recalls Bucher. He concealed the intrusive footings in the stairs around the lowered area.

The ways basements once dark and dank can be turned to full everyday use are endless. The excavation costs can run upwards of $9,000, depending on whether you use the less-expensive machine digger or have the work done by hand with pick, shovel and air hammer. The cost of finishing touches depends on how the space will be used and who does the work. But, according to Bucher, homeowners with an expansive yen can save from 20 to 40 percent of the price of building an addition of similar size simply by digging in. CAPTION: Picture 1, Capital Hill homeowners James Burk and Michael Tubbs have transfigured the front half of their once dark basement into an airy dining room that draws its light from windows looking out on a newly dug areaway and the street. The house can be entered through a new doorway leading into the dining room or through the original main entry, one floor up Tucked in the rear of the basement is the Kitchen. Moving the kitchen cleared the way for the installation of a handsome den and bar in the former kitchen area upstairs that opens to the back yard. Margaret Thomas; Picture 2, When the photograph was taken, Georgetown resident Penn Kemble was still deep into the excavation of his basement that will ultimately produce a home office and a guest room. "It was the only way I could increase my living space," Kemble said "I'm thrilled." He estimates the table project will cast him about $32,000. Margaret Thomas; Picture 3, An exterior view of the Burk-Tubbs house. Margaret Thomas; Picture 4, From the basement kitchen of the Burk-Tubbs house, stairs lead to the back yard. The grill catwalk gives access to the yard from the main floor. Margaret Thomas