The proposition that you can improve the looks of a landmark building by adding to it is highly questionable. Nonetheless, it is happening all the time in downtown Washington. Many of the more interesting, and better, new projects in the area are architectural add-ons.

I suppose in a timeless world no one would want to do such a thing. But there are those damnable practical considerations to spoil the vision--like do we alter or destroy, and who's to pay? And then cities are such composites, going in different ways at different times, that to recognize this in architecture can be a tremendously positive thing.

Architects themselves are arguing out the matter of appropriate stylistic response in a number of projects downtown, many of them in or near the city's historic financial district, which centers on the two blocks of 15th Street between K Street and the Treasury Building at Pennsylvania Avenue NW. In each case the challenge resulted from the difference between the height of the buildings as they stand, and the permissible zoning. The responses divide themselves into fairly clear categories, with some cross-over: deference, duplication, correspondence and contrast.

Deference is a matter of courtesy, something one would hope any designer would show when adding to (or even building nearby) a fine old structure. It becomes a matter of style when the new architecture strives to hide itself, as does the two-story addition on top of the First American Bank building at the southwest corner of 15th and H streets NW by the architectural firm of Keyes, Condon and Florance. It would be hard to imagine a better approach for this forceful 1906 building with its Corinthian colonnades, massive cornice and balustraded roof line.

The addition itself is a simple, clean-lined modernist shed in a gray matching stone and high, dark glass windows. The setback of 19 feet is sufficient to hide the new piece from view at most places along the street, and the detailing is sufficiently bland that even when it can be seen, it makes no great impression. The architects did allow themselves (and their clients) one bit of exuberance in the form of a rounded balcony that provides a view down 15th Street, a statement perhaps better left unmade but tastefully handled.

Copying is a form of deference, too, although it does seem a somewhat spiritless architectural approach. Still, sometimes it is the best and maybe even the only way to go, as when considering how to add two stories to the Southern Building at the northeast corner of 15th and H, a 10-story Beaux Arts pastry cake designed by the Daniel H. Burnham firm and built in 1910. From an esthetic point of view the case for two extra stories on this building seems hardly compelling, but architect Shalom Baranes did discover evidence that the original clients and architects considered the additional floors, and failed to build them not because of art but because of money.

Presumably this helped to persuade a majority of the Joint Committee on Landmarks to favor the project with their votes. Whether Baranes' addition looks like what Burnham would have done is impossible to say since the original drawings were not found, but it can't be too far off. Baranes copied his ornaments from existing terra-cotta motifs on the building, and put them together in (for this building) a plain-as-pie fashion. A crucial test will be how well the new materials will match the old. If the added piece shines with its newness, as do the newly-added floors with their poorly matched bricks on the 17th and DeSales Street facades of the Mayflower Hotel (Vlastimil Koubek, architect), then the architect's ingenuity will have been for naught.

The battle between correspondence and contrast as responses to this challenge is, in some ways, a recasting of the ideological dogfight between modernist and post-modernist. The typical modernist predilection is for a clean slate from the ground up. Lacking that, the preference will be for abstraction and contrast. This attitude is personified in the design by Mariani & Associates for an eight-story addition to the Playhouse Theater building, 727 15th St. NW, an ornate little chunk of a Beaux Arts bank building currently disguised by the red awning of an X-rated movie house.

To build a slender sliver atop this building is indeed a challenge, the more so because its immediate neighbors, especially the effulgent Folger Building with perhaps the best mansard roof in Washington, have so much character. Mariani proposed a design "to emulate the bravado and strength" of the area, and came up with a marble and ribbon-window facade articulated with rectangular bays in the middle floors and balcony setbacks at the top.

Bravado the design certainly has, but subtlety is hardly its strong suit. It is not really very sensitive to the building it uses as its base nor to its neighbors. The horizontal fenestration seems especially unfortunate, and the setbacks at the top too skimpy to avoid doing insult to the mansard roof of the Folger Building. The addition unquestionably will stick out for all time; unfortunately, this is the clear intention.

Correspondence is almost by definition a more promising tack under such conditions. Two designs by Baranes--for the Bond Building at 14th Street and New York Avenue NW, and for the Army-Navy Club on Farragut Square--illustrate the point. (Whether the Bond proposal gets built is another issue; currently it is in a sort of developers' limbo, even though it did pass muster with the Joint Committee on Landmarks.)

The Bond Building, an elaborate Beaux Arts office structure with many virtues beneath its layer of decay, has been preserved only by the strenuous efforts of preservationists, supported by the courts when challenged. Baranes' design for two thin, "bookend" buildings, one facing 14th Street, the other facing New York Avenue, and a setback stack of four additional floors on top, is an act of spirited deference to the old building. The new parts take many cues from the fenestration and rhythms of the existing structure, but they clearly read as something new.

Similar things can be said about the Baranes design for the Army-Navy Club, although what is being saved here is not a building but simply the two street facades, facing 17th and I streets, respectively. Although designed in 1911 by the notable Washington firm of Hornblower and Marshall, the Army-Navy Club was never a terribly distinguished structure. Furthermore, it was disfigured by additions on the side in the 1940s and on the top in the 1950s. These spiritless and insensitive add-ons will be destroyed to make way for the 1983 version. Spirit, sensitivity and complexity characterize this new design, which will make a much more appealing landmark than the original. That is quite a happy, unusual result.