HOUSTON -- The building designed by Italian architect Renzo Piano to house the Menil Collection is a refined, even an elegant, box. It has the air of being just the building it wants to be -- long and low, gray and white, steel and glass and wood. It sits comfortably upon a flat, spacious lawn in a low, informal residential area not far from the downtown skyscrapers.

In a time of edgy historical quotation, this museum wears its Miesian modernism with ease. (It may be just a coincidence, but maybe not, that Ludwig Mies van der Rohe's only American museum building is the Houston Museum of Fine Arts.) Piano's precision-engineered roofing system -- 300 light-deflecting ferroconcrete "leaves" held in place by cast-iron trusses -- seem as much handicraft as hi-tech. Inside many galleries and working spaces are bathed in natural light. The rectilinear rooms are white-walled and simple -- at once monastic, efficient and welcoming. It is a restful, and quite a beautiful, building.

This gentle quality, especially, is something of a surprise, coming from an architect whose most famous building (designed with English architect Richard Rogers) is the decade-old Centre Pompidou, that radical, lively, hi-tech insertion in an old Paris neighborhood. But the differences are implicit in Piano's approach to architecture: "The solution is embryonic in the commission and the site, everything is there to be decoded," he has said. Houston is not Paris, in other words, nor is Dominique de Menil, a private patron par excellence, a central government bent upon making an extravagant break with the past.

Three requirements, besides the architect's self-evident predilection for technological expression and a historical modern architecture, were particularly important in determining Piano's course. The first was de Menil's strong desire that the museum be neither showy nor monumental -- she told the architect, it has often been reported, that she wanted the building "to look small on the outside and as big as it can be on the inside." The second was light -- "I told him I wanted to be able to turn the sun on and off," recalled Director Walter Hopps.

And the third was the nature of the art collection itself and the collectors' guiding philosophy. Comprising more than 10,000 objects of extraordinary diversity in terms of period, style and size, the collection is personal, almost the opposite of encyclopedic. To Dominique de Menil and her late husband John, ready availability of the art objects to scholars and connoisseurs was at least as important as continuous public display.

Hence, just about a third of the building's 100,000 square feet of floor space is devoted to exhibitions and other public functions, a reversal of the conventional museum ratio. The remaining space is devoted to offices, workrooms, a library and other needs of the professional staff and, most exceptionally, a sequence of commodious storage rooms that are more like galleries or libraries. The art in them is extremely accessible, hung on walls or displayed in glass cases, and the rooms will be outfitted with chairs, tables and desks for staff and visitors.

These client requests and functional divisions can be read outside and inside Piano's box. It's taller, for instance, on one of its long elevations than the other; the low side contains the galleries and the higher houses the "storage" rooms and professional spaces. On the inside a light-filled hallway running the length of the building north to south separates these public and "private" areas -- a simple but inspired device.

The galleries, like houses in a village, open onto this corridor/street, and each is different from the other. There are two huge rectangular rooms, one the quintessential Miesian open space, fully sky-lit and with but a few thin steel columns, and the other carefully subdivided with temporary partitions, showing the inherent flexibility of the space and also Hopps' idiosyncratic touch as an installer of contemporary art -- round every corner there's a surprise, more often than not a pointed one. But there also are enclosed, intimate, artificially lit galleries -- little houses appropriate to the mood and scale of Cycladic figures or Byzantine textiles -- and there are glass-walled chambers for African art, seen against the foliage of interior gardens.

In itself the circulation spine is an austerely beautiful space, narrow, high and focused -- the glass walls at either end open to light and gardens, and up above, in parallel rows, the elegant leaves of the skylight dramatically reiterate the equation of light and art with nature and man, one of the principal themes of the place. Detailing here as elsewhere is superb -- air vents in the polished, dark-stained oak floor, simple grilles of the same material and color, reinforce the longitudinal sweep. They're nice to look at, and to walk on.

Piano's ability to work with engineers is of course best shown in the roof. Designed in collaboration with Peter Rice and Tom Barker of the Ove Arup Partnership in London, and manufactured in an English boat-building factory, the leaves are arranged to allow some direct north light into the interior, and curved to deflect direct sunlight (it is reflected from the top side of one to the underside of its neighbor.) Unfortunately there's already some discoloration in their surfaces, but basically they are beautiful artifacts that splendidly attain their goal -- inside the museum one is always aware of changing light conditions outside. Even the cannisters housing supplementary electrical spotlights, so obtrusive in most modern museum buildings, are here affixed in tracks on the tips of the leaves, so they're more like Christmas lights, or birdhouses perhaps -- comfortable, "natural" appendages.

The building does indeed seem to be a smoothly engineered machine -- there are lots of other ingenious touches, such as the "positive pressure" air control system that, besides providing constant interior humidity, also obviated the need for esthetically displeasing double entrance doors. (One feels the air push out when entering.) Such machines, in the past, rarely fit so well into their neighborhoods -- Mies' National Gallery in Berlin, for instance, is the ultimate in free-standing boxes (although his Houston Museum, smaller and attached to an older structure, is more polite).

Some of the reasons Piano's building is a successful contextual piece are obvious; others are oblique. There are regional references -- such as the surrounding arcade -- and the campuslike setting is important. The de Menils began buying up property in the area years ago, and it now totals 23 contiguous acres. As a result, the context is something of a set-up: turn-of-the-century clapboard bungalows, several moved from other locations, flank the building on its long sides, making a perfect "residential" foil for Piano's structure. Furthermore, these little houses (many used as museum offices) are uniformly painted gray and white -- a de Menil theme established years ago. Gray clapboard siding with white steel trim -- exquisitely detailed of course -- were thus reliable choices as his exterior materials. Piano from the start obviously had total control of scale -- his building is a perfect centerpiece in the little de Menil village.

The Rothko Chapel (another de Menil-sponsored undertaking) is right next door and helps to set a quiet, art-precinct tone. Then, too, close by there is a cluster of horizontal, modernist buildings -- most notably the main campus of St. Thomas University, an ensemble designed by Philip Johnson (with architectural fees paid by the de Menils) in the early 1950s and based in plan upon Thomas Jefferson's University of Virginia and in materials and style upon Mies. Piano, it has been said, was more influenced by postwar Los Angeles modernists such as Craig Ellwood than van der Rohe, and the fine, slender proportions of his work suggest that this is correct. But, such esoterica aside, the Johnson campus obviously makes Piano's building seem much more at home -- it's a gentle, sophisticated reprise of the '50s in just the right, unusual place.

The Menil Collection doesn't exactly look like a museum -- a casual driver-by could mistake it for a school, maybe, or a superclean factory. This is a problem inherent in the neutral, modernist box. Somehow, though, it's a negligible issue here. Up close, there is no mistaking the function -- one walk around the building under that enchanting arcade is sufficient. One can look inside and see, for instance, a framer working in his shop; even more important, one becomes sensitized to the succinct poetry of those fine details -- fitting art in an art building. After one visit, of course, the meaning of the place can't be forgotten. This is the ultimate, pleasant surprise.