IF YOU HAVE a building or even an empty lot in a historic district, you might get a sizable tax credit by promising to leave it just the way it is.I Scenic easements -- donating control over the outside appearance of your house or the development of the open space to a preservation organization or a nature conservation group -- are one way people here and across the country are preserving historic streets, neighborhoods and open spaces.

Simply explained, with a preservation easement, you give up the right to develop or change your property. The non-profit organization that accepts the right acquires an easement on the property. An appraiser judges how much this reduces the tax value of your property. The amount of reduction becomes a charitable gift, deductible from your income taxes, and perhaps your estate taxes. The real tax saving is something only you, your tax adviser and the Internal Revenue Service can calculate.

"It is a private, voluntary technique," said Thomas A. Coughlin III, assistant general counsel of the National Trust for Historic Preservation. "It enables the property to stay on the tax rolls, remain in private ownership, yet be protected. In many cases, it's better than freezing a house in time and making it a sterile, unprofitable house museum. It is an important way that communities can protect a whole street or neighborhood."

Coughlin points out that it's also a way of ensuring that one's heirs will not tear down the old homestead and build a high-rise over it.

Such an easement generally does not give the public access to your house nor limit its resale.

In the District of Columbia, two organizations were established specifically to accept such easements. (See Page 2 for similar organizations in Maryland and Virginia.)

Since 1975, the Foundation for the Preservation of Historic Georgetown has accepted scenic, open space and architectural facade easements on buildings in the historic district of Georgetown.

Since 1978, the L'Enfant Trust has accepted similar easements on buildings within all of Washington's historic districts: Dupont Circle, Logan Circle, Capitol Hill, Anacostia, Le Droit Park, Lower 16th Street, Lower Massachusetts Avenue and Georgetown. Although the trust usually defers to the Georgetown Foundation for buildings in that district, it will accept them.

Both are dedicated to preserving Washington's magic mix of mansions, rowhouses and handsome commercial buildings. Neither discriminates against a house because of age -- or lack of it. Both attach a bronze plaque to structures protected by the easements.

"We think Georgetown is a pretty great place just like it is now," said Robert F. Evans, a retired army colonel who is chairman of the foundation's easement committee. "We like the Georgetownwe've got, not the Georgetown of 150 years ago. We accept easements on contemporary buildings as well as historic ones. Low and livable, fitting into the landscape -- that's what we hope to preserve."

Georgetown has accepted 53 easements -- all houses, so far, though it doesn't discriminate against commercial buildings. "I figure that if we get 100 easements, we will have stabilized Georgetown," Evans explained. "With the easements in the right places, we can control the development of the whole block."

L'Enfant Trust has 19 easements in several historic districts.

Two easements, for instance, were given by the owners of twin 1929 Georgian Revival houses by the late Washington architect Waddy Wood, who built many Kalorama area houses. The owners are Mrs. B. Reath Riggs, the artist and a daughter of the architect, and Brig. Gen. and Mrs. James L. Collins Jr.

Richard W. Snowdon III has given an easement on the 1910 Beaux Arts office building he owns at 1607 New Hampshire Ave. NW, once the Woodbury Blair residence. The building was designed by J.H. de Sibour, who designed more embassies than any other Washington architect. Snowdon has also given an easement on 1605 New Hampshire Ave. NW, an 1892-94 Beaux Arts building designed by architect Albert Burnley Bibb.

A four-story 1891 building at 1733 Connecticut Ave. NW is protected by an easement given by owner Royce Lanier. The lower floor of the building now serves as Larimer's Market storage, with offices above. Designed by B. Stanley Simmons, the building is being converted to all offices. The 1909 Interstate Building at 418 10th St. NW, designed by B. Stanley Simmons, is soon to be redeveloped according to a design by David Schwartz. Owners Mr. and Mrs. Arthur Morrissette have given an easement on it.

Margaret S. Dean, L'Enfant Trust's executive director, explains the easements as "a gift to the street. Our covenants require that you do no harm to the exterior of the building. We don't ask that people spend the money for elaborate restoration, or even remove certain blemishes. Of course, if the building has been altered too drastically, we wouldn't accept an easement on it."

Such easements run in perpetuity -- forever. The legal agreement is similar to the easements commonly held by landowners that give one property's tenant the right to walk over the land or come in the driveway of another. The agreement "runs with the land." It forms an integral part of the deed and, therefore, is not to be lightly entered upon.

Anyone interested should first consult his real estate tax lawyer. The easements may have a decided effect on the value of the property. Some buildings are enhanced because they are counted as worthy of preservation.

But the values could be diminished because the easement prohibits additions to or alterations of the property. The major prohibition, of course, is against tearing down the building. Even if the building burns, for instance, the easement might prohibit the lot from being incorporated into a site for a high-rise. The trust or foundation would be able to insist that whatever is rebuilt on the site conforms to the original use.

If you grant such an easement, you might not be allowed to add another story to the building, stick an air conditioner through the wall, enclose a porch, make any changes in a window, take off the trim or even paint the building a different color -- indeed, alter its appearance in any way. Depending on the easement, you might not be allowed to put a swimming pool in your backyard, install a concrete deer in your front yard or even cut down a tree.

Organizations differ. The L'Enfant Trust doesn't worry about landscaping. The Georgetown Foundation protects trees of eight-inch diameter or more.

The Georgetown group also requires that the property be maintained "in a good solid manner. Otherwise, we have the right to repair it and send them the bill," Evans said. The L'Enfant Trust does not require a maintenance standard.

Neither group is concerned with interior changes that don't affect the outside.

The Georgetown group requires that one not change the basic use of the building. Thus, if it is a single-family home, you would not be allowed to divide it into apartments.

The L'Enfant Trust feels that "by allowing for adaptive, changing use of historic, easemented property, we best ensure that building's survival over time."

Both groups send around a committee every year to inspect the houses and make sure that they have not been messed up.

The Georgetown Foundation is supported by freewill donations, some by people who don't even live within its historic district. The L'Enfant Trust charges for its services in accepting easements.

The L'Enfant Trust requires a fee of 5 percent of the easement's value on an owner-occupied house and 10 percent of the easement's value on a commercial property. These fees and other expenses in connection with the easement may also be tax deductible.

In giving an easement to the L'Enfant Trust, the homeowner first hires a qualified appraiser, one from the Appraisals Institute of America, the Society of Real Estate Appraisers or the American Society of Appraisers. This cost may range from $500 to several thousand dollars. The appraisal determines the value of the easement. The owner completes an application form, either signs the L'Enfant Trust's easement deed or submits one from his lawyer and submits a $500 check as a fee deposit. Half of the $500 is returnable if the easement is not accepted.

The L'Enfant Trust then prepares an architectural and historical documentation of the property, including current photographs. When this process is completed, the committee meets to accept or reject the easement. If the easement is accepted, it is recorded on the land records of the District of Columbia.

The Georgetown Foundation accepts easements in much the same way, except that it does not require a fee, though, of course, it is glad to have donations.

For more information, prospective donors can call: Colonel Evans at 337-4456 or the Georgetown Foundation's lawyer at 659-9884, or the L'Enfant Trust at 347-1814.