Tamotsu Nakada needn’t do much to reach his neighbors: He can simply extend his arm and touch two of their houses, each of which is a mere foot from his property line, from his small terrace. “Having more light and air was important to me,” says Nakada. But when Tokyo houses are packed in like commuters during rush hour, you need a smarter brand of density.

Enter Nakada’s friend Koji Tsutsui, an architect based in the San Francisco Bay Area and in Tokyo. Designing around the property’s obstacles, Tsutsui devised a bent roof that provides an expansive sense of space for the 793-square-foot house he created for the 935-square-foot lot. By diagonally chopping off the downslope of a typical pitched roof, Tsutsui allowed more of the sun’s rays to penetrate the interiors of his so-called Bent House. The resulting form blends in with the pitched rooflines of the neighborhood while simultaneously breaking from the vernacular with its bold, sculptural profile. The roofline serves another function, shading a terrace that provides the luxury of outdoor living space within this staggeringly tight urban environment.

Tsutsui and Nakada met in the early ’90s while studying architecture at the University of Tokyo. Nakada went on to become an advertising copywriter, and Tsutsui got a job with the Pritzker Prize–winning architect Tadao Ando, whose concrete houses are known for their simplicity and engagement with the elements. “He is one of the architects I respect the most,” Nakada says of Ando. “I dreamed of creating a concrete house like his.”

That opportunity came in 2010, after Nakada had spent eight years in a dark 400-square-foot apartment. A nearby property had been subdivided, and the resulting four lots were available, each shaped like a flag on a flagpole. Nakada contacted Tsutsui, who by then had his own firm and was dividing his time between Tokyo and the Bay Area—where he had become the Joseph Esherick Visiting Associate Professor of Practice at the University of California, Berkeley, College of Environmental Design. Unsurprisingly, when it comes to design, the former schoolmates are kindred spirits.

“We already shared the same aesthetic,” says Nakada, sitting with Tsutsui in the 292-square-foot, sun-filled living-dining area of his home one recent afternoon. “His work always has a specific concept. He’s good at minimal houses. And he knows how powerful simplicity can be.”

Tsutsui has been experimenting with the idea of houses as an arrangement of boxes since starting his own firm in 2004. “The way boxes come together is really interesting to me,” says the architect, who creates geometric models from Styrofoam and museum board to help him conceptualize spaces. “We adjusted the combinations to see what kinds of spaces they created in relationship to the site and the surrounding buildings,” his associate Satoshi Ohkami explains. After trying out some 20 combinations, they finally transferred the selected design to a computer.

Tsutsui describes the arrangement of this house as a collection of light wells: “The main box brings light into the main space upstairs; the other, small ones bring light into the first floor.”

The main volume of the house, with its bent roof and two skylights, consists of the kitchen and living-dining area, with a bedroom and bathroom beneath it. The second box holds the entry, with another bent roof that harvests sunshine for the first-floor hallway while providing protection from rain at the front door. The third brings the sun’s rays to the first-floor study.

If Tsutsui gave his friend the house of his dreams, he did it by inviting heavenly light from above into an earthbound, unadorned piece of architecture. Where bolder statements have been made, they appear with an evenhanded elegance reminiscent of Tsutsui’s mentor. The sole stairway consists simply of Oregon pine timbers cantilevering out from a wall (the planks are reinforced with steel). On the lower level, the bedroom sets itself apart from the study and bathroom with a plywood wall that stops just below the ceiling, creating a box within a box and a sense of openness.

Though Nakada found ways to save money on this project—the plywood cabinets in the kitchen are one example—he didn’t scrimp on everything: Splurges include the foot-wide Oregon pine flooring used throughout and a Duravit sink for the bathroom.

Nakada kept the furnishings minimal and minimalist. The main living area contains little more than Finn Juhl chairs, a vintage Alvar Aalto dining table, and a vintage Radiohus pendant lamp by Vilhelm Lauritzen. When Nakada works at home, he spends most of his time in this room, where light pours in from the south-facing bent roof and the skylights atop the 12-foot ceiling.

From his terrace, Nakada’s proximity to the adjacent roof has no effect on his sunny outlook. Living alone in a city where 300-square-foot studios are the norm, all this light and air is a luxury. “Within what looks like a closed concrete box, I can feel the four seasons,” says Nakada. “If it’s sunny, I go outdoors to feel the wind.”