“We liken ourselves to a 19th-century guild,” says Katherine Newman of the Toronto-based firm she cofounded with architect Peter Cebulak in 1991. Because rather than specialize in a single field Katherine Newman Design spans multiple disciplines, from architecture to interior design, construction management, and even landscaping.

This unique bandwidth allows the organization to take on a wide variety of projects, from large scale—they’re currently developing Brooklyn’s tallest building, Brooklyn Tower—to more intimate residences. “We take on only five projects a year, but our body of work is very diverse,” says Newman of her lean and nimble methodology. “We have no stamped aesthetic and hire no junior designers. We see ourselves as editors.”

That’s why an international financier, after purchasing a landmarked 18,000-square foot Georgian Revival–style residence in Rosedale, one of Toronto’s most historic neighborhoods, approached the firm in the first place. As he and his family would be living in London before moving in, they needed someone they could not only implicitly trust, but who also possessed a deeply rooted understanding of how architecture influences design.

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The goal? To dovetail a timeless aesthetic with dynamic tension: “We wanted to capture prewar elegance and postwar modernity,” Newman says. In addition to sourcing furniture and carefully scaling everything to each room’s individual dimensions, her firm designed custom pieces “inspired by French, Danish, and Italian midcentury designers, with a focus on furniture as art.” The resulting collection was fabricated by artisans from all over the world.

Since a renovation from the 1990s left the structure feeling rather drab—“there were inadequate sources of lighting, and the use of weighty colors (including a dining room painted black), negated the vastness of the space,” Newman explains—her firm ushered in a litany of striking light fixtures and softer hues. These also help complement the homeowner’s ever-growing collection of pieces by Canadian artists (like Jean Paul Riopelle and Marcelle Ferron) and photographers (think Hiroshi Sugimoto and Henri Cartier-Bresson).

Appearances aside, the house ultimately had to fit certain functions. The family has three teenagers, and regularly entertains. That often means hosting fundraisers, luncheons, and dinner parties, during which bottles from their extensive wine collection are served. “The house could be likened to an English manor house,” she says. “It possesses a certain casual elegance and lacks pretense, but is loaded with beautiful things.”