On a sunny spring afternoon in L’Oise, the bucolic French département an hour north of Paris, it was a pensive pleasure to be alone in the room of a stranger. Brief though this interlude in the main salon of Cordelia de Castellane’s country house may have been, it allowed an unfiltered experience of the aesthetic of the artistic director of Dior Maison and Baby Dior, an impressively accomplished Parisian creative and businesswoman with a multi-layered ancestral pedigree of some of the most venerable aristocratic families in Europe.

Imbued with an air of just off-centre bohemian good taste, the walls are hung with wallpaper – a delicate floral motif printed on a vanilla-custard background. Inside an antique Italian secretaire, its glass-paned doors open, are shelves containing a pair of almond-green milk-glass vases and other hand-blown designs from Murano filled with apricot-coloured peonies. Overhead, the arms of the metal chandelier hanging from the white-painted, wooden-beamed ceiling are wrapped in faux ivy. On a Napoleon III wool rug with floral motifs inside repeating Bordeaux squares, a pair of Moroccan stools flank a glass coffee table piled high with art and garden books; framed botanical and animal drawings hang on the wall. Exuberant, worldly and casually elegant, even if every detail has been carefully mused and meditated over, this mise-en-scène offers a glimpse of the vivid and unbridled imagination of de Castellane.

Cordelia de Castellane at her house in Villeneuve-sur-Verberie in L’Oise, France

“For me, there are no rules,” she says of her aesthetic and her work. Wearing a flowing embroidered hostess gown and teal-blue velvet ballerina shoes, de Castellane is immediately warm and welcoming. “I’m very traditional but I’m also spontaneous. I’m never nostalgic but I respect the past and make it part of the present. And what I like most of all is to twist things to remake them in a new way.” She points to the new Blue Mizza Dior Maison dinner service she’s just designed as an example. “When I changed the colour of a classic leopard print to blue, this well-known motif became something else entirely.”

The 18 drawings of plants and flowers on the neighbouring wall are the work of Korean artist Stella Sujin, who is represented by the British artist Magda Iris. “I love them for their storybook feeling,” de Castellane says. “Actually, this whole house has sort of a children’s-book personality, because it’s such a gentle, friendly place, a family sort of hideaway in the country that has a hodgepodge history. It isn’t all posed and fussy, because perfection is so boring. Part of it dates back to the 15th century, and may have been part of a monastery, and the added-on bits reflect all of the people who’ve lived here.”

The garden’s borders, parterres and topiary were redesigned by British landscape architect Milan Hajsinek

Though de Castellane doesn’t mention a particular children’s book, Robert Louis Stevenson’s A Child’s Garden of Verses from 1885 immediately comes to mind. “The house sort of chose us, really. I think houses do that,” she says. “My husband and I had been renting a little house on this property for a long time, because he golfs and I ride,” she explains of how the family, who also have a house in Paris, came to own the country bolthole. “Then, after the death of the owner, textile designer Primrose Bordier [who introduced France to the then daring concept of colour in domestic linens], my father told me we should buy it. He was right. It’s our haven and happy place, where we come on weekends and spend the month of July before going to Greece for August.”

“Let’s go for a little wander,” she continues, as she leads us to the front hall. “I bought this grey-and-white checked fabric, which the French call Vichy, at the Marché Saint-Pierre in Paris,” she says gesturing to the walls. Located in the 18th arrondissement of the French capital, this popular fabric market is a local favourite for its very affordable prices on fabrics by the yard. “And why not? Today there’s so much good houseware design at all price points,” she says with a smile and a shrug.

Set on a five-acre estate of groomed gardens and a tennis court, the three-storey main house has six bedrooms and six baths, along with two salons and a kitchen with a black Aga and antique tile backsplash over the sink. The floors throughout the ground floor are in tomettes – the rectangular or hexagonal terracotta tiles once common in rural French houses. De Castellane and her husband undertook a major renovation that rendered the house more comfortable and also converted the dining room into a second salon. “The way we live here, we had no need for a formal dining room,” she explains. “So instead we eat in the kitchen at a table with built-in seating and several Napoleon III chairs from the flea market in Paris, next to the big hearth of the old chimney.” The house she formerly rented is now an elegantly decorated three-bedroom guesthouse that’s the object of persistently sought-after invitations from family and friends.

“I think you learn to see from what surrounds you as a child,” she says, adding that her Greek mother worked as a preferred interior designer of the Greek jet set and is also an artist. “My mother always says, ‘Don’t be timid! Push through to what it is you’re really trying to say when you design a room.’ Both of us love colour and textiles.”

The designer, however, doesn’t see herself as having decorated the house. “I’m just sort of rearranging all of the things my family and I have collected, and perhaps making them fresh with occasionally offbeat new juxtapositions of colour and patterns,” says de Castellane, who grew up in Switzerland before spending much time in Paris with her uncle, the late Gilles Dufour, Chanel designer Karl Lagerfeld’s longtime right-hand man. “I grew up in fashion. I just found it so interesting,” she says, recalling how she had to persuade her doubting parents to let her go to work as a lowly junior “picking pins up off the floor” in the atelier of the late Paris-based fashion designer Emanuel Ungaro at the age of 16. Wisely, they agreed.

“Along with my mother, Ungaro’s wife Laura was my mentor, the one who fed my confidence and encouraged me to set my imagination free,” says de Castellane, identifying the beautiful magenta and blue florals on a black background fabric on one of the two sofas in the second, smaller salon of her house as an Ungaro design. This cosy room is also adorned with an elegant gilt-framed mirror that belonged to Boni de Castellane, the marquis who was a famous Belle Epoque dandy and Cordelia de Castellane’s great-uncle.

De Castellane worked with Ungaro and his associate Giambattista Valli for several years, and finally left to take some time as the mother of small children. She never really stopped working, though, since she founded a beautifully designed, reasonably priced line of childrenswear under the CdeC (Cordelia de Castellane) label, which she ran for 10 years. “My idea was that there had to be something better to dress your children in than Gap, because they grow so quickly.”

In 2012, Christian Dior recruited her to design its collection of children’s fashion and she went to work at the venerable brand’s headquarters on the chic Avenue de Montaigne in Paris. Five years later, they tapped her to become the first artistic director for Dior Maison. “I was both flattered and intimidated,” says de Castellane, who decided to accept the job because “I love the visual wit of Monsieur Dior. He was a flawlessly elegant man who also had a perfectly calibrated ability to be politely iconoclastic.”

Available exclusively at Dior’s flagship store on the Avenue Montaigne in Paris, the home collection has been an insider favourite among design connoisseurs for decades. Now it’s on sale for the first time at the Dior store in Miami’s Design District, with plans in the wings for corners in the brand’s Beverly Hills, Houston, New York City, Washington DC and Vancouver stores. Dior Maison is also sold at the Dior boutique on New Bond Street in London.

“I like to think of Dior Maison as sort of a big bazaar, or an exciting place where everyone will find something to love because it caters to all tastes and occasions,” says de Castellane. “This is why I’m so pleased by our new line of willow baskets – bread baskets, waste baskets and others. They feature a star design, because Monsieur Dior often made reference to his lucky star, and the craftsmanship is just so beautiful. And I like to think that Monsieur Dior would also have been pleased by our new Limoges porcelain dinner service with the motif of lilies of the valley, because he loved them so much. His mother grew them in the garden of the house where he grew up in Granville in Normandy, and he often wore a lily of the valley in his lapel.”

Her hope for her own designs is that people will still be using and loving the objects 100 years from now, “because beautiful things withstand the test of time”, she concludes. With an instinctive talent for provoking the traditional codes of good taste with originality and irreverence, de Castellane’s designs are doubtless destined to become classics in their own right.