Turning into Passage Buhan from the main road Rue Montmejean through tall black wrought-iron gates, the hustle and bustle of the Place Stalingrad, the nearby public square with its busy intersection and criss-crossing tram tracks, is soon forgotten. The lane comes to an abrupt end at Mr. Buchholtz’s converted garage.

The old storage space is unrecognizable today. Where there was once a forbidding metal barricade as the front entrance, there is now an open glass and honey-colored pine facade that cost just over 9,000 euros ($12,044) to create. To keep out prying eyes, a sliding pine door on aluminum runners can completely cover the new double-glazed glass portion of the facade.

The pine door slides back to reveal an open-air interior patio with a teak floor that allows both natural light and ventilation into the apartment. The sparsely furnished patio, while creating an attractive entrance hall, also brings the property into compliance with Bordeaux city laws. “The authorities require every property to have a parking spot for one vehicle,” said Mr. Buchholtz, who doesn’t own a car. “The patio acts as our official parking spot. It’s the exact size required by law for such a place.”

From the patio, the living area is reached through another sliding glass door. This area includes a white Formica and chrome Ikea kitchen and a dining area with a glass-topped table. To give the studio apartment a more expansive feel, Mr. de Marien designed a “house within a house,” a gleaming cube-shaped construction of Douglas pine that dominates the far end of the living area.

“By combining all the factors of living in this self-contained cube — such as sleeping, washing, working and storing — we kept all of the necessities of Jérémie’s life in one place,” the architect said. “By building upwards towards the roof and internally into the heart of the cube, we had no need to spread these aspects throughout the living area.”

The cube, which is 11.8 feet long, 9.8 feet wide and 9.8 feet high, was built at a cost of 9,389.50 euros ($12,565) by the Bordeaux-based furniture designer Julien Lamire.

Concealed inside the wooden cube are the toilet, shower room, laundry and a dressing room. On the cube’s exterior, a wheat-colored organic sofa bed is recessed along one side, next to a small office space separated by a thin wooden divide. Along the opposite side of the cube are narrow steps leading to the sleep loft, as well as the door to the bathroom. Upstairs in the sleeping area, the bed is built into a deep recess in the cube construction, surrounded by custom-made storage cupboards and bookshelves. The bed is embedded within the cube, instead of perched on top, to stay in compliance with historic preservation codes. The architects were prohibited from increasing the garage’s height because the roof is part of an original 18th-century structure. A skylight above the bed offers Mr. Buchholtz a view of the stars and welcome ventilation on balmy nights.

“I’m a person who doesn’t need a lot of space but I do need the space to work for me,” Mr. Buchholtz said. “This is functionality by design but apart from that — it’s just a funky, cool place to live.”