When Linzi and Andy Knight moved their animation and design studio from London, England, to Toronto, Canada, they knew a bigger family was in the plans and that living close to the studio was essential. The couple bought a home on a north-facing lot in the center of town. The location was perfect—but the house was not.

Seven years and three children later, the Knights were ready for a change. They knew exactly what they wanted: more light, fewer walls and absolute serenity. Enter Martin Kohn of Kohn Shnier Architects, whose repertoire of clean lines and simple design attracted the couple. "We knew he would create something wonderful," says Linzi, who nonetheless kept Kohn and his associate, Maggie Bennedsen, on their toes with her own keen design sense.

And so the entire third floor gave way to a private wellness sanctuary for these busy parents. The long, narrow bedroom-cum-bathroom retreat was created simply by dividing the rooms with an elegant two-way fireplace. Dark merbau floors, warm walnut cabinetry and travertine lend drama to the serene setting, while the clean palette makes this urban oasis appear larger. The spa bathroom extends out to a covered balcony overlooking the densely treed backyard, which offers privacy as well as abundant natural light. The upstairs retreat is distinctly open, since the children are welcome in the physical epicenter of their parents' private lives. In fact, the only door in the room leads to a glassed-in water closet and shower offering a hidden spa-like retreat, replete with a bench set under a rain shower head and surrounded by jet and steam taps. It's in here that the couple can soothe away the frenetic pace of work and family life. With the open, Zen-like tone set by the third floor, the design for the rest of the home follows naturally. On the second level, three bedrooms, a bathroom and a laundry room are fused together by one central lounge area. A curtain wall—a stunning mahogany-framed window—spans the entire width of the house and extends down to the first floor, ushering in nature and light. Below, the flow from living room to dining room to kitchen is seamless; storage issues are solved by sleek walnut and mahogany built-ins, offering a hiding place for the inevitable clutter that comes with family.

While the pitched roof presented a challenge, Kohn managed to keep the room long and rectangular: The shower, W.C. and cabinetry all run the length of the bathroom's walls and offer privacy from the main open room, as well as a clever way to hide the eaves.

The entire home's design reflects total simplicity and calm. Still, when Linzi and Andy need to escape, they head to their top-floor sanctuary—a special wellness space that ultimately makes this house a peaceful family haven in the center of a bustling city.