The leafy settings that make many California hillsides attractive to homeowners, can also make the locations challenging to build on.

Such was the case for owners of a ’50s-era single-story house who asked San Francisco architect Cary Bernstein to build an 1,100-square-foot master suite on an unused eastern portion of their lot. The house, which sat downslope of a densely wooded hill, enjoyed privacy and a postcard view of the bay. The adjacent plot was level and seemed ideal for an addition they had always wanted: a serene personal retreat with a spa-like bathroom.

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The site, however, was unstable, with a history of landslides. So Bernstein, a local architect, worked with her team to devise a deep pier and grade beam foundation to bypass the loose soil. They embedded more than 30 concrete piers 8 feet into the bedrock before any construction on the addition could begin. As an extra precaution, they erected a 50-foot-long, 6-foot-high concrete retaining wall uphill to buffer the new volume from future landslides.

The existing structure has 8-foot high ceilings, typical of the era in which it was built. To create the contemporary feel the clients desired, Bernstein devised a master-suite wing with a 10-foot-high ceiling in front of the retaining wall, bridging old and new with a small, 9-foot-high interstitial structure to mediate the variations in height. Glazed doors on its north and south sides provide alternate entry points to the house.

Inside the suite, a double-sided limestone fireplace screens the bedroom from a scene-stealing 340-square-foot master bathroom, which Bernstein wrapped in thermally broken low-E windows to let in daylight and expansive views. “In all of our projects, we try to have daylight enter a room from at least two sides,” she says.

An acrylic soaking tub occupies the center of the space, flanked by a pair of 16-foot-long floating vanities mounted on facing walls. A mirrored wall above one of the vanities reflects the tree-lined views of the window on the opposite wall. A spacious walk-in shower and private water closet are positioned back to back at the east end of the wing. Although the clients were motivated primarily by aesthetic interests, the shower’s zero-threshold entry, low integral bench, and handshower conveniently positioned for a seated user, are all designed to accommodate aging in place. Thanks to full-height glazing here, even the bench is immersed in the view.

While the addition succeeds in inviting the owners to slow down and soak up the setting, the architect made sure it still feels dynamic. “It is even more lively as the sunlight moves across it throughout the day,” Bernstein says.