This colourful Notting Hill house received an elaborate refresh in just 12 months

If you didn’t know better, you might fully expect Mary Poppins to skip down the steps of the London townhouse that a young couple and their growing family now call home. The early-19th-century terraced Regency abode, with its white stucco exterior and classic black door, would look right at home on Cherry Tree Lane, where Mr. and Mrs. Banks (Poppins’s employers) lived and the indefatigable nanny worked her magic. Indeed, it took some serious sorcery by international interior designer Kate Guinness and her eponymous firm to transform the dated interiors of the 3,400-square-foot Kensington house within the one-year end date that was agreed upon—at which point, the family of three would become four. The arrival of their first child had shrunk their Notting Hill flat as it was; it was time to spread out.

Guinness is no stranger to sleight of hand. The designer’s team pulled the proverbial rabbit out of their collective hats by delivering on their clients’ wishes for a colorful yet practical and functional home within 12 months, right down to the art on the walls. “And we moved a lot of walls,” she says. It is a skill that comes quite naturally to the London- and Wiltshire-based designer. In an earlier life, Guinness designed costumes and sets for theater and opera. “Quite often, I was looking to distill sets—and costumes for that matter—into key elements, items that worked hard to telegraph the who, what, where, when, and why,” she says. “And we could easily move walls to get the look we were going for.” It was great training for working with permanent spaces.

Among the boldest moves was to flip the ground floor, which traditionally houses the kitchen, with the second level. “It was the husband who suggested this, and as counterintuitive as it may seem, it makes a lot of sense,” says Guinness. On the ground floor, where there was once a pass-through from the kitchen, there is now a handsome bookcase that encloses the drawing room. Beyond it are a boot room, a powder room, and a sitting room. Guinness gutted the next floor up to include an open-plan kitchen, dining room, and sitting room. It’s no surprise that this is where the family spends most of its time.

MAY WE SUGGEST: Kate Guinness fills a formerly minimalist Kensington house with joyful colour and pattern

But the owners would admit that every square inch of this house is alluring, not least because Guinness has done what she does best: filling every room with elegant layers of color, texture, and pattern. It all starts with those walls. The designer leans toward timeless colors, the kind you might find in historic houses, which is the point. “My hope is that my clients never become bored or tired of them. On the other hand, I didn’t want colors that would overwhelm them either,” she says. Lighting followed, and that’s where the team tapped into their founder’s experience with stage performances. “I learned so much about how light impacts everything, from the paint colors, to furnishings, to fabrics—and not least, the feeling in a room,” she says.

Furnishings and curtains came next, followed by choosing fabrics, art, and lamps. (Cramer & Bell served as the art advisory.) For Guinness, playing around with textiles is genetic; her textile-loving mother filled her childhood home with them. “They do two seemingly contradictory jobs by connecting all the elements in a room and at the same time throwing the whole thing off ever so slightly,” she says. Just like Mary Poppins herself.