THINK ABOUT remodeling the Washington monument into a high-rise condominium. You could put balconies up and down one of the sides, with a viewing section, perhaps for a posh restaurant cantilevered out from near the base. Certainly you'd want a nice piece of abstract sculpture atop the entry. What about some swooping wings encircling the building to serve as carports? Landscape it nicely with bushes and trees -- whamo! What a scene!

The Washington Monument project -- a 5 1/2-foot particle-board sculpture -- is a fantasy of local artist David Stainback, who has some other dreams (nightmares?) about other Washington monuments -- including the "Jefferson Memorial Restaurant" and the "Lincoln Memorial Bank."

Models and sketches by Stainback and 19 other American architects and artists, are in the exhibit: "Architectural Fantasies: Creative Alternatives." It opens Tuesday to the public and continues through Jan. 25, 1981 at The Octagon, 1799 New York Avenue.

The American Institute of Architects Foundation organized the show. The projects vary between wild extravaganza, pie in the sky projects and imaginative, but workable designs.

SITE (Sculpture in the Environment), a New York architectural firm, has actually built its fantasies. At their newest in Richmond, Va., for Best Products Co., you can't see the building for the trees.

The 66,000-square-foot Best building invaded by trees is -- what else -- a Best Products showroom. The grand opening was in October.

The $5-million Best Products Co. building stands on the edge of a dense forest on Quioccasin Road, in the West End section of Richmond. The facade is reasonably normal, but look again, and you'll see that a forest of oaks, 50 feet high and more, actually grows up through the 25-foot-high building, slicing off the facade from the building proper. As you come in the front of the building, the trees grow in an atrium between the two sections. An alternative plan showed the building wrapped in trees, but that version was declared not practical.

The project is just one more of the hilarious buildings perpetrated by SITE Projects, an architectural firm that has been egged on with money by Best Products Co. The first SITE-Best building in Richmond has a corner that looks as though the brick veneer is peeling off -- very unsettling.

Best Products Co., one of the bigger American patrons of architecture as art, has its headquarters in Richmond. They will soon have a national headquarters in Richmond in a building designed by Malcolm Holzman of Hardy Holzman Pfteiffer. The Best buildings were recently the subject of an exhibit at the Museum of Modern Art. A Best building by Robert Venturi, as well as imaginative architectural fantasy showrooms by SITE, are also in Houston; Towson, Md.; Miami, and Hialeah, Fla.

Best's Washington-area showrooms are relatively ordinary. "Best doesn't really ask SITE to do a building to fit," a company spokesman said. "We just show them our new sites; and if they feel like doing a building for it, they do." Every architecture firm should have such patrons. Officers of Best have said they find the imaginative buildings draw crowds, so good art can be good business.(The principals of SITE are James Wines, Allison Sky, Michele Stone and Emilio Sousa.)

Sydney and Frances Lewis, Best founders, also have endowed the Virginia Museum of Art at Richmond with a fabulous Art Nouveau collection. It is significant that the Lewises are interested in both Art Noveau and American abstract and fantastic art, because one is certainly an outgrowth of the other.

Few architecture patrons are as imaginative, rich and free-handed as the owners of Best Products Co., a principal sponsor of the new show.

But SITE is not the only firm that has found an enlightened, or at least a playful, sponsor. Ted Curtis, an Ohio architect, kept looking at the 36 abandoned Quaker Oats grain silos in Akron and thinking of ways to use them. Now, with financing from the Quaker Square Management Company, the silos (which once held 1.5 million bushels of grain) have been re-modeled into 144 24-foot round luxury hotel rooms, named the Quaker Square Hilton Inn.

Some of the projects are not as serious. Two of the pen-and-ink sketches are from David Macaulay's "Great Moments in Architecture," published recently by Houghton Mifflin Co. A Macaulay sketch is the poster for the show ($10 signed, $5 unsigned, with proceeds going to the AIA Foundation endowment). The drawing shows the "inflatable Cathedral," which according to its designer "creates a distinguished setting at a moment's notice." His other design is a supersized cinder block from "the Garden of Architectural Delights." The cinder block has a series of hanging gardens at different levels and sits upon a dais rimmed with pillars and a wall.

Dickson Carroll, another Washington artist who moonlights as an architect, has made a model/sculpture he calls "Uptown." It could be a building, Carroll thinks, for "the quiet and dignified elegance of aristocratic, old-world-charm Fifth Avenue, opposite Central Park."

Carroll planned it in the spirit of the older buildings that "dramatize the experience of entering." A sunken entrance is covered with knobby towers, surmounted by a canopy that looks as though it might fire off to the sky any minute. Two more winsome columns hold up a section surmounted by what looks like elephants holding up a block of a building with space port windows. The tippy top of what would be the main structure seems to be a dovecoat with doves serving the same purpose usually carried by carytids.

Carroll's other two models/sculptures in the show are called "Downtown" and "Midtown." Carroll likes to walk down the street and imagine that real buildings are now, or perhaps once were, themselves models for something larger.

The AIA/Octagon show includes some older architectural marvels including "Lucy the Margate Elephant." Lucy is one of three elephant-shaped buildings erected in 1881 along the East Coast by James V. Lafferty, a real-estate developer. The other two have long ago gone to the elephant graveyard, but Lucy has been a National Historic Landmark since 1976 and is open to the public. The show also includes the "Tail of the Pup" hot-dog stand from Los Angeles.

Pop artist Claes Oldenburg alleges that all of his projects -- if he found someone with enough money -- would be buildable. The question is why? In the show is an Oldenburg proposal for a cathedral shaped like an enormous faucet to pour forth a Niagra of water, planned for a site in Lake Union, Seattle.

Oldenburg, for years, has thought about buildings in the shapes of three-way electrical plugs. He made plug sculptures in the mid '60s. And he has designed an electrical plugshaped crematorium, a chapel and a museum extension.

In 1967, Oldenburg drew a clothespin scaled to skyscraper size and titled it "Late Submission to the Chicago Tribune Architectural Competition of 1922." This spring, Stanley Tigerman with Stuart E. Cohen and Rhona Hoffman took Oldenburg's spoof one step further. They organized an exhibition called "Late Entries to the Chicago Tribune Tower Competition." Both the original competitions and the spoof competitions have been printed in a two volume set by Rizzoli. Robert A.M. Stern's model of his entry is included in the Octagon show. The design, suggested by Viennese architect Adolph Loos' entry in the original contest has a huge signboard around the top. Flat pilasters rise in a black-spandrel-glass and white-frosted-glass design. Not included in the show, but in the two volume set encompassing both competitions, are some funnier ones. William Turnbull Jr. proposes a simple facade with no building. Hans Tupker suggests a tower shaped like a Tommy gun. Donlyn Lyndon drew a tower topped with a T-shirt. Livesy/Rosenstein a giant baby's bottle -- and so on.

The 1922, $100,000 international competition for a new administration building for the Chicago Tribune attracted 204 designs from architects in 23 countries. The winning design was by John Mead Howells and Raymond M. Hood. The runner up was Eliel Saarinen.

Architect Stanley Tigerman takes an imaginative, but still architectural, approach to fantasy design. His "Bathroom in the Intention of Dante's Inferno" in the Octagon show doesn't pretend to be anything other than a building -- a hell of a building. Undoubtably, its classical illusion comes from the time Tigerman spent at the American Academy of Rome. Tigerman would like the Kohler Company to build the bathroom, or rather bath house.

The bath house is designed to be built of water-filled glass block, with silicone joints and "Trombe" walls, as they're called, designed to heat water with the sun. Hot and cold plunges are at either end.

Tigerman says the idea comes from Dante -- Absolution. So each fixture is given its own "altar-like" space. "The metaphor for hell," says Tigerman, "is the steambath which one descends into from the peripheral purgatorial edges of the little building. The heavenly metaphor is couched is a chaise longue underneath a cloud painted ceiling."

Late entries in the Octagon show are expected from other well-known architects, including Philip Johnson, Paul Rudolph, Paolo Soleri, Richard Ridley, Harry Weese & Associates and Cesar Poelli.

Early in life, childhood's favorite toys, the building sets, become the way the child learned about the transitory nature of life, the fact that all work can eventually come to naught.

Castles of shifting sand are impregnable only until the next tide comes in. Their defenders dissolve before the onslaught of the tides. Building blocks stacked with the greatest attention to balance and structure are powerless before the casual kick. Skyscrapers of Leggo blocks, strongly interlocked in mimic of brick, can be disassembled as well.

Most architectural fantasies are as ephermeral as buildings of sand, block, or Leggo. But they have their uses nevertheless. With their own mad logic, they point out that laughter is functional, too.

The Octagon show comes at a time when architects, principally those who call themselves "Post Modernists," though they should call themselves Pop (and Mom?) Architects, are moving more and more into actually building fantasy structures. Putting in columns that don't hold anything up, skylights at angles that suggest the sky is falling, using pediments that don't ped -- all these self-conscious attempts to jazz things up are becoming common among the avant garde set.

All architects, even the Internationalists with their cry of less is more, are dedicated to aesthetics as well as function. But the dividing line between art and architecture has to be the proportions of utility. Today, with the shortage of oil, the rumors of war, the prevalence of volcano eruptions, the decay of the cities and the inflation of building costs, more than ever we need buildings that keep us warm, happy, and safe. It would be sad if architects, when faced with such problems, just go off in a corner to play.