Montreal’s Old Port, lined with brawny cranes and fine 18th- and 19th-century facades, offers the perfect tableau of a traditional river city. But a glimpse across the St. Lawrence River reveals Habitat ’67, towering in the distance like the building blocks of a playful young god. The building stands equally tall in the city’s architectural history: Conceived by architect Moshe Safdie when he was a 23-year-old student as a model of prefab housing, the residential complex was a standout at the city’s 1967 world’s fair, known as Expo ’67, and is now a landmark of architecture in Canada.

Designer Byron Peart, originally from Ottawa and cofounder of the leather goods and accessories brand Want Les Essentiels de la Vie, had wanted to live there “since forever, basically,” he recalls—ever since he first visited Montreal as a child. Recently, he and his partner, the German-born fashion executive Stefan Weisgerber, realized that dream when they bought one of the 148-unit building’s apartments: three concrete “cubes” that had been gutted and left unfinished by the previous owner. The couple turned the two-level space into a polished home that brings Safdie’s creative modernism into the 21st century.

For the enormous crowds at the world’s fair 45 years ago and ever since, Habitat has been a manifestation of the potential of prefabrication. It’s comprised of box-shaped concrete modules that were built at a nearby factory and lifted by crane into three irregular pyramids. The modules interlock in a variety of configurations, producing units of varying sizes and levels. Each apartment has one or more outdoor terraces, which are located on the roofs of adjacent units.

Aiming to create a more humane model for the inner-city high-rise, Safdie envisioned a building that would offer many of the amenities of a suburban house: lots of light, fresh air, gardens, and outdoor “streets” on which residents could gather and mix. One of Safdie’s mentors from architecture school, working as an Expo ’67 organizer, hired him to work on the fair’s master plan. And—in a sign of that era’s experimental spirit—the organizers then commissioned the young architect to oversee the construction of a design based on his student thesis project. In the end, budget reductions forced Safdie to downscale his plans, and a 22-story section with a planned mix of uses, including a school and retail spaces, never came to pass.

Still, when Weisgerber and Peart went apartment hunting, they found the complex remarkably well preserved. The concrete exterior bears some water stains, but each architectural detail—from the planters along the elevated walkways to the broad pyramidal fountain—is more or less intact. (It helps that Safdie still owns a place on the top floor and keeps a watchful eye on the complex.) Though some of their neighbors’ units feature pink wallpaper and chintz drapes, Weisgerber and Peart wanted their new home to harmonize with the building’s original architecture. “The challenge was to make ours a modernist unit that was timeless and classic,” Peart says. “We wanted it to feel representative of what [Safdie] wanted the building to be.”

To the duo, that meant an interior influenced by the greats: Scandinavian design, Mies and the Bauhaus, and American mid-century industrial design. The couple started with a raw shell. Interior designer Maria Rosa Di Ioia of Idea Design worked with them to rebuild the apartment with a palette of white, black, and oak that sets a backdrop for their colorful collection of art, books, furniture, and vintage Expo ’67 memorabilia.

From Peart and Weisgerber’s front door, you can admire river views and many of the building’s unique details, including entryways scattered up and down staircases and cantilevered concrete rooms. The entrance to the apartment leads to a double-height space, formed by two stacked concrete cubes; the bedrooms are to the right, while the living room is upstairs in a higher-level box. Just off the entrance is a kitchen with white Bulthaup cabinetry and in the opposite direction, a dining area crowned with a passerelle, or footbridge, made of slatted white oak and black steel, that stretches through the top half of the space. “We couldn’t leave all that space unused,” Peart says. The passerelle leads to a spectacular set of bookcases, from the German manufacturer Cubit, which mimics the intersecting boxes of the building’s architecture.

The thoughtful interplay of scales is a device that makes the interior work well, despite the unconventional layout. The upstairs living room is about 625 square feet, with enough space to dock a boat-size plush B&B Italia sectional. But it is the windows, tucked into two corners of the room, that dominate, offering unbeatable views of Montreal’s Old Port and skyline. A massive fireplace of Noir St. Laurent marble counters the view with some dark-hued heft. This combination of glass and marble reminds Weisgerber, a well-read and devoted fan of Bauhaus architecture, of Mies’s work. “It’s our tribute to the Barcelona Pavilion,” he says, of that famous building—a labyrinth of reflective glass, marble, and water—from the 1929 International Exhibition.

Though four decades have passed since its construction, Safdie’s architectural vision remains intact. Habitat ’67 is still a place where forthright modernist forms, lovely vistas, fresh air, and intimate promenades are all part of the everyday experience of living. When Weisgerber and Peart arrive home, removed from the traffic of the city, they escape into an environment where the architectural ideals of the 1960s are still, within their concrete walls, alive. “This place is a refuge,” Weisgerber says, “and we have worked hard to make it that way.”