An envious mood must occasionally strike anyone who drives Highway 101 past the Sausalito houseboat district on Richardson Bay. You can't help but think how pleasant such a home would be, combining all the cosmopolitan amenities of the Bay Area with a close relationship to the natural world. After a busy day at the office, you could go home, crack a beer, watch the rafting scaup and goldeneye ducks and listen to the grunts of the mating toadfish: bliss!

Still, traditionally there hasn't been much scope for architectural expression on a houseboat. Many of the Sausalito houseboats are small and exceedingly modest constructions, though some are far more ambitious, bordering on the luxurious. But all, ultimately, are of a type: boxes on barges or pontoons. Sometimes there's one box, sometimes a series of boxes cunningly joined - but they generally do not push at many boundaries, architectural or otherwise.

Until now. Enter Tafoni, a wholly different kind of houseboat proposed by Joanna Borek-Clement, a 30-year-old Polish-born San Francisco designer. Her houseboat, which is characterized by rounded shapes and open, light-filled interiors, takes its name from coastal sandstone formations marked by eroded, smooth-walled concavities. The sensuous, almost organic shapes of the tafoni near Salt Point on the Mendocino Coast exerted a deep attraction for Borek-Clement, and it occurred to her they could be incorporated into a residential structure.

"I've just always been fascinated with rock formations," said Borek-Clement during a recent interview over a couple of Bass ales. "I get really absorbed by the polished stones you find along the beach or a river. And when I'm in Las Vegas, I spend more time at Red Rock Canyon (a state park noted for its sandstone spires and cliffs) then I do in the casinos."

Further informing her houseboat project was an admiration for Japanese interior design.

"There's a freedom of shape and form in Japanese interiors, and a corresponding de-emphasis on color, that I find very attractive," she says. "It really allows you to think of living spaces in a different way."

Indeed, Borek-Clement characterizes Tafoni more as a "floating pavilion" than a typical residence. The emphasis is on the continuity of space and light rather than on discrete rooms.

The primary structural elements, ellipsoidal wooden trusses, provide a large spanning capacity, eliminating the interior walls and columns that can create cramped conditions in many houseboats. By virtue of their location and the ample use of plate glass, most houseboats present a pleasing panorama of nature, but Tafoni takes it to the next step, creating the illusion that the living space is part of the natural environment - without the downside of actually living outside in the wind, rain and fog, of course.

Tafoni has some green elements, as is obligatory for any serious architectural project these days. The floating pontoon that supports the houseboat can be made out of a variety of materials including fiberglass, but Borek-Clement prefers prefabricated concrete, which is relatively inexpensive and environmentally superior to the usual alternatives. The ellipsoidal supports, she notes, can be fabricated from recycled wood or plastics. She also feels that houseboats in general have fewer impacts than terrestrial dwellings because they are not permanently fixed to the land, minimizing habitat fragmentation, compromised water quality and soil erosion.

Still, Borek-Clement acknowledges that Tafoni is at essence an exercise in aesthetics, not environmental do-gooding. And building Tafoni would be well beyond the budget of most back-to-the-land - or rather, water - enthusiasts. As Borek-Clement has designed the prototype, it could cost several million dollars.

"It's all custom work, and there's a lot of curved glass in there," she says. "That all adds up. But costs could be brought down through any number of means, including some redesigning and economies realized from scaled production."

Tafoni isn't Borek-Clement's first sally into unorthodox architecture. She also designed Sky-Terra, a plan created for an architectural competition that is perhaps even more striking than Tafoni. Consisting of a series of domed plazas perched atop tall, soaring towers, Sky-Terra is pegged for high-density urban areas such as Tokyo's Roppongi District. Because Sky-Terra employs a design that is the opposite of the Egyptian pyramids - a wide top and narrow base - it required sophisticated conceptualizing to ensure structural stability.

"Dealing with the natural forces that resist a design like this was very challenging, but I enjoyed the whole process immensely," Borek-Clement says. "I work for a design firm in San Francisco, and I like the rigor of contributing to pragmatic, commercially viable projects. But my private efforts like Tafoni and Sky-Terra are really creative exercises - I don't want to restrict myself."

Both Tafoni and Sky-Terra have generated quite a bit of chatter in the blogosphere, negative as well as positive. Borek-Clement takes both the good and bad reviews lightly. What pleases her more than praise, she says, is stimulating discussion.

"I'm not claiming I have any solutions," she says. "That's not my goal. I see my projects as question marks, as conversation starters. I'm saying, 'Look at this - what do you think? What are your ideas?' My projects involve large concepts, but they're not going to change the world. I'd be very happy if some of them result in small civic improvements - or even some well-considered critical judgments."