Every year the Washington Metropolitan Chapter of the American Institute of Architects makes historic preservation awards. The program is worthwhile if only to make us think about the subject.

Many people talk a good line about "preserving the nation's architectural heritage" but their projects don't always achieve that aim.

To cite a few: the people in Alexandria who want to hang federal false fronts on a huge modern courthouse: the builders filling in vacant lots in handsome Victorian rows with federal facades: the developers who want to tear down a handsome Adams-Morgan mansion to replace it with dozens of rowhouses with itsy-bitsy passed bay windows.

Preservation is complicated. You can't just go through the lumber company catalog and pick out a few cornices and carriage lamps and call it preservation. You have to give deep and long thought to such essentials as scale, color, use. That's the problem with the courthouse in Alexandria, as Washington Post writer Eduardo Cue pointed out. You can't hide a 246-foot-long office building behind fake shutters.

Besides the matter of size and scale, the new buildings or additions need to be compatible in color - you wouldn't add a green polka-dot sofa to your silvery-blue brocade living with bad habits such as those which smoke or are noisy , would make everybody happier if they lived alone.

Sadto say, the local AIA chapter hasn't turned up too many great preservation achievements this year, in contrast to some years before. The jury, chaired by M. Hamilton Morton Jr. couldn't find anything in the Metropolitan area it thought worthy of a first award. To console themselves, the AIA members are having their awards ceremony Thursday in a building that won their 1976 award, the Smithsonian's Arts & Industries building, restored by the Smithsonian's office of facilities, planning and engineering services with architect Hugh Newell jacobsen as consultant.

But they did select two designs they counted as worth merit awards, significantly both additions to historic buildings, rather than true restorations. The merit winners are the Folger Shakespeare Library remodeling and addition at 201 East Capitol St. SE, by George Hartman and Warren Cox, and the Robert Elliott House at 17 West Irving St., Chevy Chase, Md., also by Jacobsen.

The Folger addition includes a new two-story underground book stack addition beneath the parking lot, and a new reading room addition, placed over the existing terrace within the U-shaped court at the back of the building. Remodeling will also include some rearrangements as well as a new airconditioning system with humidity control.

Judging by the sketches, all we have to go on at the moment the new reading room will be pleasant enough, with canopies under a skylighted ceiling to give soft light. Entry will be through three-story high sky-lit courts, which will also save the existing windows at the old building and thus the natural lighting.

The rear, the only visible side (from the alley) of the new addition, is planned to be rather plain Jane, nothing to imitate or compete with Paul Cret's magnificent art moderne 1932 facade. The building should be pleasant, workable, and cause no hard feelings in the neighborhood.

The Elliott House by Jacobsen is one more of those elegant jobs that this architect is so good at doing. The new addition, a living room and a master bedroom, is designed as a tongue-in-check "eminent Victorian." The old section was made to match Jacobsen's idea of a proper 1871"English-style" house. the two sections are linked with a hansome 1977 glass entry. Jacobsen chose the colors for the house to reflect the Victorian fondness for three-color effects.

As usual in Jacobsen houses, the detailaing inside, especially on the bookcases, is equisite.Other qwards go to two modest projects: the Metro-Chiller Building at 519 13the St. SE by Perry, Dean and Stewart (consulting architect, Harry Weese & Associates), owned by the Washington Metropolitan Area Transit Authority; and the National Bookstore in the National Visitors Center, owned by the Parks and History Association of the National Park Service, designed by Hartman/Cox.Both of the two citation award winners were contracted by George Hyman Construction Company.

The Metro-Chiller Building was built in 1901 as a blacksmith and wheelwright shop. The building to house Metro machinery was chosen because of "its size, location and adaptability . . . in harmony with the immediate neighborhood." What better reasons could be given.

The bookstore project sets handsome modern black blocks with brilliant red interiors in the white neoclassical space. The units are functional and the color is cheerful and a pleasant relief from the all-enveloping white. (Even though one patron suggested that the red bookcase interiors made him think it was an "adult" bookstore.)

When giving credit on preservation, it's easy to pass over the executors of the design.

Today, we are once again beginning to apreciate form for form's sake. Who knows in a few years, we may be brave enough to ornament a few new buildings. We should nurture the craftsman for that time.

In the meantime, projects like the local AIA chapter's awards help to remind us that it's better to keep what we've got rather than try to reproduce it.