Designed by the architect Mark Dziewulski, with the garden and pond by Haag Landscape Architecture, the clean, simple-looking marriage of water, concrete and glass was a complicated undertaking that belies the extension's setting. Tucked away behind hedges, bushes and trees are suburban backyards and bungalows.

The client's wife, who grew up in Hawaii, has long loved water and tropical plants. In 1970, the couple bought a house in a residential neighborhood on the banks of the meandering Sacramento River, the city's natural preserve. They commissioned Ed Haag, a landscape architect, to build a koi pond in the courtyard.

Two years ago, the couple bought a neighboring parcel and began planning a large garden that, Mr. Haag said, would extend into water. Thus was born the idea of creating another, bigger pond. But the existing house - a mid-60's ranch house with three wings flanking a courtyard that opened onto the river - had no view onto the new parcel. Mr. Dziewulski presented two schemes, one an extension in the style of the existing house, the other much more striking - an open pavilion shooting out from the house and up from the pond, over the water and toward the river.

The clients took the plunge and went with the second option, giving the architect the latitude to create a modern sculptural form. They were captivated, the wife said, by its "wow factor."

In the manner of Cubist-inspired buildings like Rietveld's Schroeder House and Le Corbusier's chapel at Ronchamp that feature visually distinct interlocking vertical and horizontal planes, Mr. Dziewulski's design consists of seemingly discreet elements - the soaring roof, the west wall with irregular fenestration, the south- and east-facing wall of glass - interconnected by what he calls the proscenium arch of supporting concrete piers and a steel I-beam.

It looks effortless but, as Mr. Dziewulski said, "making things look simple is so hard." Building the pavilion "was as complex as building the space shuttle." The stucco-covered steel-framed roof, designed for maximum lightness, does not just stretch upward, but also widens and curves as it extends over the water. And it tapers near the tip to appear lighter.

Meanwhile, the 1,200-square-foot extension blends with the existing 10,000-square-foot house. Sand-colored limestone flooring flows from the old to the new space. And the new roof swoops up from the downward slope of the house's Cliff May-style wide-pitched roof. Mr. Dziewulski said, "There are just enough elements like the big overhang of the sweeping ranch-style roofs" - echoed in the soaring pavilion roof - "to make the visual connection."

Facing east toward the river, the glass wall is folded into translucent, clear glass panels, allowing a view of the river while providing a backdrop for contemporary, New Guinea and African sculptures. Panes had to be delicately inserted into stainless steel channels that would allow for movement due to wind loads, heating and cooling. Such a feat of construction involved coordination with the glass manufacturer (Commercial Window Systems), contractor and structural engineer, Mr. Dziewulski said. Intense cooperation between the architect and the landscape architect was also key to the integration of pond and pavilion.

A whopping 6,000 square feet, and eight feet deep in parts, the pond was built like a swimming pool with six-inch thick Gunite walls. To meet stringent codes, the footings for the studio were built first, then the pond shell was poured. To clean the pond, which laps innocently at the soft edges of the surrounding garden, requires a concealed pump that recycles water 24 hours a day.

The project was unusual because of the generous proportions of the garden and pond, which fill an entire lot. These days, Mr. Haag said, gardens are getting smaller and buildings are getting larger. Most gardens, he added, are not as lavish as this one, which is a veritable Monet's garden with lilies, irises, lotuses and plump goldfish.

Ponds, Mr. Haag said, are costly to maintain, use a lot of water and have no function. Aside from cooling the air, they simply provide delight. This pond is in fact shamelessly, hedonistically, contemplative.

But what delight! At night, frogs bust into song; ducks and deer have wandered from the river; blue herons have homed in and, true to their natures, tried to stab the goldfish. Moreover, the water dapples the inside of the pavilion and mirrors the lights of the building. The landscaping in and around the pond offers color and texture for every season. In spring, a Western redbud tree offers a lush cerise contrast to the stark gray-green limestone of the pavilion.

Mr. Dziewulski's clients were so delighted with their extension that they have virtually moved into it. They entertain, watch television and eat dinner there. The wife, who is active in the arts community, also works there at a desk that extends into the middle of the room from a wall of neatly crafted shelves, drawers and filing cabinets. The extension beckons with light and views and tranquillity, so much so that the original house now feels like the add-on.

Mr. Dziewulski trained at Cambridge University and has worked for Michael Graves, for the corporate modernists Skidmore Owings & Merrill and the English firm Ahrends, Burton & Koralek. Three of his recent projects - this pavilion, a Virgin Megastore in Sacramento and Davis Commons, a nearby retail center - won American Institute of Architecture awards last year.

In this pavilion, Mr. Dziewulski wanted to build an exuberant jewel in the landscape, a radical, architectonic form, in contrast to its pastoral setting. While some see the spirit of dynamic 50's modernism (in the manner of Le Corbusier or Oscar Niemeyer) in his pavilion, he is not a diehard modernist. The seeming simplicity of the style suited this project, he said, but each commission demands something different. "Each one makes you reinvent yourself," he said.