If only every architect spent as much time thinking about the underside of a staircase as Cathi House does. “I always sculpt the undersides of my stairs,” says the co-principal of San Francisco's House + House Architects. “It's the most neglected opportunity, but it's really the place to do the most exquisite detail. Because there aren't code issues, you can do whatever you want.”

It's perhaps no surprise, then, that this geometric gem is one of her personal favorites. “Generally, it's just a stair,” House explains, but in this Northern California residence “it becomes the railing of the bridge upstairs, the flying shelf in the living room, the cabinetry. The way the materials wrap around and come together guides you in how to move through the space.”

Designed as sculptural pieces with tolerances “so perfect you need a cabinetmaker to do it,” the staircase comprises warm, elegant woods—Santos mahogany, anigre, and ebony—that were cut, finished, and installed by San Rafael, Calif.-based IK Design to stringers built by the project's general contractors, Innovation Builders and Fulwiler/James of Emeryville and El Granada, Calif., respectively. The panels' forms, like the materials, are echoed throughout the house: in detailing on the nearby back door, for example, and in the horizontal slats that grace the skylit second-floor corridor. “I wanted there to be a rhythm to it,” House says of her design, “to give an overwhelming sense of tranquility to a complex shape.”