WHEN IS A BUILDING NOT A BUILDING? Once, there were orderly boxes known as buildings. But what's this? Ramshackle designs with slanted porches and asphalt shingles are breaking the rules of structure and composition

AT first glance, the model of the Familian House looks a little like the illustration for a headline: ``Million-Dollar Mansion Hit by Hurricane.'' It is a spacious, California-looking place - it should be sun-drenched and set by an infinite blue ocean. It does not, however, look secure. Mostly that's because its long, white sides are crisscrossed by unpleasantly slanting and sinister toothpick porches.

Still, this design, by innovative architect Frank Gehry, is riveting; you can't look away from it. You gape at it dubiously - you want to walk around and around it. If it were a car, you would surreptitiously kick its tires.

The buildings in the Deconstructivist Architecture exhibit at the Museum of Modern Art look like a hallucination. The catalog bluntly compares them to a nightmare: ``They are structurally sound, at the same time they are structurally frightening.''

Fragmentation is the mood here. Form is ``contaminated,'' ``distorted,'' ``violated.'' Distortion is not decoration; it is part of the design. A model of a blue, wooden skyscraper (designed by Coop Himmelblau, of Austria) twists toward the ceiling. One end of a mammoth bar (by Polish-born David Libeskind, now living in Italy) rests on the ground; the other slants up to a height of 10 stories in the air. It so defies your idea of what a building should be that this non-expert humbly assumed for a while that the beautiful giant models were some kind of decoration.

``I don't understand,'' a woman said to herself, a little desperately, as she looked at an architectural drawing labeled ``Section showing example of use of open space: a cinema.''

In the next room, several gray-haired ladies were figuring out, with great interest, the floor plan of ``Rooftop remodeling 1985'' (also designed by Coop Himmelblau). This roof, presumably an ordinary structure at one point, had been given, among other things, a wing poised across the top, like a hat with a springy, fat feather. A less low-slung and substantial building would have looked as if it could take wing itself, if it had a mind to.

The exhibit is a little confusing. There isn't much space between things. You triumphantly manage to relate the model of the Familian House to its blueprint on the wall over yonder. And a minute's careful scrutiny of the many squiggly lines of ``Section showing example,'' etc., reveals what might indeed be a cinema.

You go out to the sculpture garden to commune further with the catalog (by curator and architect Philip Johnson), and learn that deconstructivism isn't a movement; that the seven architects represented here do not communicate or work together. In their different corners of the world they have come up with buildings that argue with most people's idea of what a building should be.

In the catalog, Mr. Johnson evokes a famous and prophetic exhibit he designed for the Museum of Modern Art in 1932, ``Modern Architecture,'' starring Mies van der Rohe, Gropius, and Le Corbusier. What Johnson thinks of as the icon for that exhibit is a photo of a shiny ball bearing - which certainly stands in as the inspiration for the trim, tidy, glass-skinned skyscrapers we're all accustomed to today. The ``icon'' for the deconstructivist exhibit is a photo of a fallen-down shed, titled ``Spring house, Nevada.''

It is a vivid and creative way of describing the difference between the modern skyscrapers and the buildings in this exhibit. Still, you see the collapsed shed, its weathered boards flopped out in an exhausted-looking way, and imagine a millionaire staring at it as an architect says, ``Here's what I can do for you.''

You leave the museum with the horrible, mashed-nose-against-the-windowpane feeling that you are left out, that you had missed the point.

It is always fascinating for those of us who don't deal with it all the time to get a chance to think about what is beauty and style. Artists and architects are always a few steps ahead of us; we waddle behind, trying to catch up, looking at the blueprint and saying, ``I don't understand.''

Coincidentally running at the Whitney Museum is an exhibit of the work of Frank Gehry, one of the architects in the Museum of Modern Art exhibit. Mr. Gehry is a Los Angeles architect famous for his intuitive designs, use of low-cost industrial materials, and for bridging the gap between art and architecture.

The exhibit opens with a metal walk-through fish, at a slight angle, as if the fish were leaning into the water; inside are Gehry-designed snakes and lamps, lit from within to show the pattern of the scales. Nearby is a model of an office building inspired by the shape of a fish, a sort of narrow, flat-topped, sand-castle shape with overlapping glass windows.

All of a sudden, the idea of a building being inspired by a fish rather than a box doesn't seem quite so far-fetched.

In the next section, nicely displayed in their own ziggurat, are some more models - with photos of the building and explanations, both of which are helpful to non-experts. One was the Norton house (1983-84), in Venice, Calif. A small, boxlike shed on a pole sticks up in front. On the side you see a stairway of sky-blue tile; behind it, a wall of glass revealing a red metal fireplace and chimney. It looks like pieces of several different houses stuck together.

In a way, it's like the kind of houses you see all the time in Maine - the ones that look as if grandpa had built a house of white clapboard, the son had added a green-shingled garage, and then a nephew started to paint the house yellow five years ago but so far has only finished one side.

Except that this hodgepodge was designed by someone of such taste and subtlety that it looks terrific without looking meant. It feels like a beautiful juxtaposition you discovered yourself.

The explanation for this syncopated design is that it's difficult to design a building to stand out in an area of crazy, ramshackle beach houses: ``Anything you put in Venice is absorbed in about 30 seconds - nothing separates itself from that context. There's so much going on, so much chaos....''

The next house on display, the Benson house, 1981, consists of two buildings covered with asphalt shingle, kind of Mutt and Jeff in proportion and style. It challenges those of us whose only idea and ambition, upon moving into a house with asphalt shingle, would be to replace it with white aluminum siding. You realize you might have been narrow-minded about this; after all, Picasso made a bull's face out of an old bicycle seat.

Like the Norton house, this is a playful design. The tall, brown house contains the kids' room on the ground floor, the parents' room above; the low, green house has the kitchen/living room area. Wooden stairs and bridges connecting the two give it the feeling of a playground. You can imagine the kids playing king of the castle on the roof, which is to have a log cabin just for them. Meanwhile, a closet with secret staircase in their room goes up to the roof, where the cabin will be.

The wildest house in the exhibit is Gehry's own, an ordinary pink bungalow that peers out, like a hostage, from behind corrugated metal and chain-link fencing. The fencing and the windows in the house twist and turn. One was inspired by Duchamps's ``Nude Descending a Staircase.'' It would be very odd to buy or sell a house like this. It's so organic; it looks as if it would grow around you, become part of you.

It definitely does not look like your basic suburban home. Apparently Gehry's neighbors think so, too. In the catalog, ``The Architecture of Frank Gehry,'' the architect says his neighbors have come over to say, ```I don't like your house.' And I say, `What about your boat in the backyard? What about your camper truck? It's the same material, the same aesthetic. They say, `Oh, no, no, that's normal.'''

The catalog also explains that Gehry got interested in chain-link fencing when a client wanted a very secure weekend house:

``When you talk about using chain link, people are filled with horror. But if they have a chain-link fence around their house and you ask, `Why aren't you filled with horror about that?' they say, `Oh, that's just a fence.' Somehow there's an idea that if it's inevitable, it's OK, but if it's a conscious or intentional use of the material, it's somehow threatening. ... When I went to the chain-link manufacturers to ask them for information about the material, they said, `Listen, we sell more of this stuff than we need to, and we're not looking for new ideas.'''

Why is it that the reaction to anything new to us is negative? We run screeching for the exit, demanding something at which we can nod, in a familiar way, ``Yes, yes.'' But sometimes the thing you dislike the most has something to teach you.

You look again at the picture of the Familian House and your imagination stirs; you can't imagine why you didn't immediately realize how interesting it is, how it expands possibilities. What is previously irritating becomes desirable, something to yearn for.

The eye adjusts, reluctantly, to a dazzle of light.

The Deconstructivist Architecture exhibit will be at the Museum of Modern Art until Aug. 30; the Frank Gehry exhibit, which originally appeared at the Walker Art Center in Minneapolis, will be at the Whitney until Oct. 2.