Located on a steep hillside in San Anselmo, Andrew and Elena’s house is hard to get to, let alone build upon. But the remote feeling of the forested setting is what inspired the couple, who are based in San Francisco, to purchase the property as a weekend retreat—even though it needed a lot of work. The dated 1950s main house was billed an “investment opportunity” and was accompanied by an unfinished ADU.

Elena and Andrew, both of whom work in the tech industry, decided to contact the ADU’s architect to help complete it. As it turns out, the architect was Casper Mork-Ulnes, who has a long history designing small dwellings. In 2004, he founded a company called Modern Cabana, specializing in prefab backyard structures, which he later sold. “Fitting a lot of stuff into a small footprint is an interesting problem for us,” he says. The original client asked Mork-Ulnes and his team to create an ADU on the 312-square-foot foundation of the house’s garage, and that it include a separate bedroom for his daughter. The architects also had to contend with the steep hillside behind the garage, which limited the site’s exposure to natural light.

The design handles both constraints, carving out both light and space. The ADU is essentially a studio apartment, and necessities for living are carefully organized along two walls, including a Murphy bed. But it also has a small loft area that can function as an additional bedroom. The roof soars upward to peek over the crest of the hill, bringing light in through operable clerestory windows.

The architects also added a deck that extends the ADU’s limited space outside. Because Andrew and Elena got involved before the ADU was completed, they were able to make some modifications to the original design. They opted for a dishwasher instead of a stacked washer and dryer and installed an under-the-counter fridge in lieu of a full-size fridge in favor of more counter space. To give them even more counter space, the architects designed a kitchen island on wheels. When it’s parallel to the rest of the kitchen, it’s an island, and when it’s perpendicular, it’s a dining table. Instead of a front door and tiny foyer, there is now a built-in sofa and storage cubbies, and the entrance is through the sliding glass doors.

The couple plans to renovate the main house with the help of Mork-Ulnes Architects and spend most of their weekends in the ADU with their two young children, who share the loft bedroom.

“I always wanted to get a camper van, and this house is like one,” says Andrew. “It’s very compact, and everyone can spend the night packed together in the middle of nature. It’s very fun.”