When Sophie Pellerin and Serge Carpentier, a couple from Montreal, moved in together, they’d been dating for five years. At first, co-habitation seemed to them more trouble than it was worth. “My condo was too small for Serge,” Ms. Pellerin said. “My house was not contemporary enough for Sophie,” Mr. Carpentier said.

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The backyard of the home of Sophie Pellerin and Serge Carpentier.

Adrien Williams

They considered living apart and sharing a weekend getaway in the Laurentians, but in November, 2016, Mr. Carpentier found a Montreal property too good to pass up: a two-storey “plex” in the Rosemont–La Petite-Patrie borough, steps away from Little Italy and the Jean-Talon farmers market. The building had a footprint of 1,000 square feet and contained three flats, two up top and one below. Mr. Carpentier bought it for $750,000 and hired Microclimat, a young architectural firm, to convert it into a single dwelling. He’s a professional contractor, so he managed the build himself. (Ms. Pellerin is a television producer who worked on the hit Quebec procedural 19-2.)

From the beginning, the couple were clear about their needs, even if the needs themselves seemed hard to reconcile. They wanted a lofty, cozy home, conducive to both togetherness and solitude, with large windows and a strong sense of privacy. If the brief sounds contradictory, well, that’s humans for you: We like sunlit rooms but hate to feel exposed. We’re introverted and extroverted depending on our moods. And having descended from cave dwellers and soil turners, we crave both open spaces and hidden nooks.

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An interior stairway connects what had previously been separate units.

Adrien Williams

The classic Montreal “plex” is adaptable enough to meet these demands. In its purest form, it is a brick box with an exterior stairway leading to a second-floor walk-up. Inside, a load-bearing wall runs lengthwise, like a spine, through the centre of the structure, which is vastly longer than it is wide. The plex is to Montreal what the gothic cottage is to Toronto or the dingbat house is to Vancouver: a form so basic it is easily overlooked, yet so common it defines the streetscape. You see it often, even if you rarely notice it, and you’d miss it if you moved somewhere else. “The typology is versatile because it’s simple,” said Olivier Lajeunesse-Travers, a Microclimat co-founder who led the design alongside his colleague Maggie Cabana. “There are just so many things you can do with a two-storey box.”

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A 12-foot-high chandelier by Montreal firm Lambert et Fils hangs over the dining table.

Adrien Williams

Out of deference to the neighbourhood character and local ordinance, he chose, however, to do almost nothing to the façade, short of removing the stairs. Inside, though, the changes are dramatic. Mr. Carpentier and his subcontractors excavated the basement, took out the load-bearing central wall on the ground floor and replaced it with a 1,600-pound steel beam, and installed an interior stairway to connect what had previously been separate units.

The floorplates are still mostly intact, except above the dining space – located at the back of the house, behind the stairs – where the second floor gives way to a double-height ceiling. A 12-foot-high chandelier by Montreal firm Lambert et Fils hangs over the table, and large windows look out onto a multitrunked silver maple tree. Both elements – one new, the other old; one fabricated, the other natural – reinforce the sense of verticality. So, too, do the white-oak slats that form a kind of wall behind the stairs and adorn the dining-room ceiling.

The architects sought to both accentuate and offset this spacious effect. “You see so many modern houses where all the rooms are completely open,” says Mr. Lajeunesse-Travers. “These spaces make for beautiful pictures in magazines, but it’s sometimes difficult to imagine yourself living in them.” The kitchen (which is just off the dining room) and the living space (which sits in front of the stairwell) are both modestly sized, with nine-foot-high ceilings. They’re cozy but still feel connected to airier parts of the house.

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The kitchen features quartzite countertops and light-blue lacquer cabinetry.

Adrien Williams

The design takes a similarly balanced approach to fenestration. The back windows on both floors are recessed 7½ feet, a feature that directs gazes, limits sight lines and allows for ample balcony and deck space. “Everybody wants light,” Mr. Lajeunesse-Travers said, “but open views are not so comfortable when your neighbours can see you making coffee in your dressing gown at 7 a.m.”

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One of the bathrooms features a standalone tub.

Adrien Williams

Perhaps the most impressive balancing act is between old and new. The material palette – white-oak floors, quartzite countertops and light-blue lacquer cabinetry – bespeaks cleanliness and modernity, but the house bears traces of the cramped prewar dwelling it once was. Upstairs, you can still discern the dividing lines between the two narrow flats that previously ran lengthwise through the building. Mr. Carpentier’s room is in what was the northern unit, while Ms. Pellerin’s occupies the southern half. A narrow hallway, which, in the past, contained mechanical systems, connects the two parts of the house.

The couple makes no bones about keeping their own bedrooms, and in this respect, they’re hardly anomalous. A recent study from Ryerson University revealed that as many as 40 per cent of Canadian couples sleep separately, an arrangement with many advantages, of which better-quality sleep is surely the most significant. Ms. Pellerin is a night owl – “I work a lot, often late,” she says – while Mr. Carpentier is an earlier riser, and both are upfront about their need for personal space. “We still visit each other from time to time,” Ms. Pellerin jokingly said.

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For two people who were once happy living separately, the house offers a model of what contemporary, urban co-habitation can look like. It is connected to the outdoors but removed from the streetscape, and it both channels and transcends its vernacular architectural history. The couple haven’t given up on their dream of a cottage in the Laurentians. They’ve bought property on a mountain overlooking a forest and commissioned Microclimat to handle the design. The build will be finished next spring. They look forward to making memories there – together and alone.