If you own an old building on Capitol Hill, in Georgetown, or in any of Washington's 10 historic districts and think you may want to take advantage of the rehabilitation tax credits allowed in the new tax law, don't touch a thing before talking to the District's historic preservation office.

By becoming familiar with the "strings" attached to the credit, you may save yourself time and money. More to the point, you may avoid inadvertently disqualifying your rennovation work for the credit.

This message was hammered home here by Interior Department officials this week at a conference cosponsored by the National Trust for Historic Preservation and Interior's National Park Service. It was organized to spell out the workings of the Economic Recovery Tax Act of 1981 as it affects old buildings.

"ERTA," as the new law has come to be known, eliminated the 10 percent investment tax credit allowed on the cost of rehabilitating commercial or industrial buildings 20 years or older. Starting Jan. 1, the law replaces the old credit with a 15 percent credit for renovated buildings between 30 and 40 years old. It allows a 20 percent credit for buildings over 40 years old, and a 25 percent credit for properties certified as historic or located in a historic district.

Unlike ordinary deductions, which are simply applied to one's taxable income, the tax credits apply to the tax itself and are thus more valuable. Moreover, they may be carried back three years or forward for as many as 15 years. In other words, if you can't take full advantage of the credit next year, you may save it and apply it to your tax bill several years hence.

Renovated historic properties used for any purpose -- commerical, industrial or rental housing--qualify for the 25 percent credit. The owner of a historic house may even occupy one unit of a town house and take the credit on units rented out.

While President Reagan's commission on housing recently recommended that nonhistoric buildings used for housing be made eligible for the lesser credits, they qualify now only if put to commercial or industrial uses.

Although the law clearly allows the taking of 15-year, straight-line deductions on one's rehabilitation costs and the 25 percent credit on those same costs, officials are divided over whether one may also take accelerated deductions on the shell of a building.

But tucked away in a law is a little-publicized provision that may be a pitfall for renovators. In effect, that provision says: If you renovate a certified historic structure, or simply a building located in a historic district, in a way that does not meet Interior Department standards, not only will you lose the 25 percent credit allowed for historic properties, but you won't even be able to take the smaller, 15 or 20 percent credits allowed for nonhistoric buildings.

Because so much of the District's real estate lies within historic districts, this rule may come as an unpleasant surprise to a lot of people.

LeDroit Park, Georgetown, Capitol Hill, Logan Circle, Dupont Circle, Uniontown in Anacostia and large parts of Pennsylvania Avenue, 16th Street and Massachusetts Avenue are all historic districts.

Ward Jandl, an official with the National Park Service, which certifies building for the National Register of Historic Places, told the group of developers and tax attorneys assembled in Philadelphia that a building inside a historic district is presumed to be historic. So unless you obtain an official "certification of nonsignificance," you must follow Interior's rehabilitation guidelines to get any credit at all. Many developers think they can avoid the hassle of dealing with government simply by forfeiting the 25 percent credit and taking the lesser credits allowed on nonhistoric properties.

If you plan to renovate and take the credit, find out first whether your building is on the National Register or located in a historic district. This information is available by calling Lucy Franklin, chief of the District's preservation office at 535-1294.

That same office can provide you with a copy of the Interior Department guidelines on rehabilitation. These are designed to make sure that a property's distinctive features are kept intact during the renovation. They prohibit such things as sandblasting because it irreparably damages the brick, the repointing of masonery with portland cement, and the use of aluminum siding. If you violate them, forget the credit.

Another "string" to be aware of is the "substantial rehabilitation" test. A controversial provision in the new law, this says that the cost of rehabilitation must exceed $5,000 or the "basis" of the building, whichever is greater. Translated, that means you must spend as much money on the renovation as you did to buy the building itself. On the other hand, if you've owned the property for a number of years and reduced its basis through depreciation deductions, you may be able to meet the test with a much smaller outlay of funds.

Developers and tax accountants who have worked out the numbers believe ERTA was incredibly generous to old buildings. As Robert Holz, with the Laventhal and Horwath accounting firm put it at the conference, "the numbers are staggering."

But for all its preservation provisions, the new law is not perfect, contends Nellie Longsworth, president of Preservation Action. Longsworth told the conference that her organization, which fought for the tax credit in the first place, is already planning to seek new changes in the "technical corrections" legislation that inevitably seems to follow major tax laws. Among them: repeal of the "substantial rehab" test that effectively excludes small projects from taking advantage of the credit; eliminate the rule that properties in historic districts must take the 25 percent credit or nothing at all; and eliminate the rule that prohibits developers from deducting demolition costs when they tear down historic properties.