During the Belle Époque, the resort city of Royan, in southwest France, was a magnet for the high-society set. Blown to smithereens during World War II, the town was rebuilt in the 1950s by a clutch of high-minded architects from the nearby Bordeaux architecture school who were under the spell of Brazilian modernists like Oscar Niemeyer. They brought to the buildings curves, abstract forms, and reinforced concrete, scandalizing the conservative Royannais and alienating longtime visitors. The retooled town quickly sunk into a scruffy postwar obscurity and has only recently reemerged as a modishly offbeat spot to own a beach house.

That’s where Florence Deau comes in. She has channeled Royan’s midcentury heritage and her own sharp eye for contemporary design into an influential blog, Flodeau, and an in-demand interior design practice. She is now at work transforming one of Royan’s emblematic modernist buildings, the former city planning office, built by the architect Yves Salier in 1952. It is an impressive showcase for any designer: sleek, white, concrete, with a glass curtain facade that curves along with the shape of the street and a vast balcony to take in the seascape. Deau has created a swank little wine bar on the ground floor and, on the top floor, a three-bedroom vacation apartment for a local wine merchant that’s a tribute to the ambience and history of Royan itself.

“It had to be inspired by the 1950s, it had to be fun—not too crazy—and it had to have a soul,” Deau says. The design touches are eclectic, from textiles festooned with tropical patterns to Slim Aarons’s pool party photographs. “We’re not in California,” Deau says. “But summertime here can feel a bit like California.”

Deau retained certain original elements in the 1,300-square-foot space, like the minimalist wood doors—made of sapelli, a reddish African wood popular in France in the 1950s—and the linear limestone fireplace. She gutted others, like the linoleum flooring and, to improve circulation, a wall separating the kitchen from the living area. The space was uninhabited for 20 years before she discovered it—dirty, abandoned, with big cracks in the ceiling and, Deau says, “lots of potential!”

The designer kept the walls white or off-white and let the details do the talking. “Light was of the essence,” she says. “I wanted to keep it simple and let the rooms be filled with colors through the textiles, the accessories, and the furniture—not from the actual architectural space itself.”

Her furniture choices say a lot about the way she works. She loves high design, be it pedigreed vintage items or gallery-quality new furniture, but she’s keenly aware that most of her clients are not millionaires. Her solution: To spend wisely, carefully blending high and low, contemporary and vintage. “You have to vary the prices so you can make your budget,” she says. “But I really like that mix—original vintage things with things that are made today.”

In the bathrooms, Deau has paired faucets by the German manufacturer Hansgrohe with simple Ikea ceramic sinks. In the living room, Habitat’s budget-minded Balthasar sofa is grouped with a vintage English coffee table, a Lalique crystal ashtray, and an original Jean Prouvé brass light fixture that Deau discovered in the apartment and meticulously restored.

To pull off this high-low two-step, Deau sources from far and wide: flea markets, local antiquarians, international design fairs, and boutique shops online— like Baan, for Thai handicrafts, and Galerie Møbler, for vintage Nordic furniture and accessories. She also conducts meticulous research. The only paintings in the apartment are by the Argentina-born French artist Nina Negri, a somewhat underappreciated Surrealist whose work, as Deau discovered, sells at a bargain compared to her male counterparts.

Deau also dabbles in DIY projects to accent interiors affordably. Beneath a Charlotte Perriand sconce and next to a quilted armchair by Noé Duchaufour-Lawrance, she has fashioned a little side table by affixing an ordinary Moroccan brass plate to a tripod stand that she found in a local hardware store. “That cost five euros,” she says with a rare flash of pride. A Gubi Grasshopper floor lamp—a modernist touchstone by the Swedish designer Greta Grossman, whose work has been exhibited at the Museum of Modern Art—stands next to it. “That did not cost five euros.”

Deau says the designers she covers on her website—from India Mahdavi to Patricia Urquiola—invariably creep into her work. “My taste doesn’t change. But after weeks and months, maybe you’ve seen a piece of furniture or a material or a color that you love. You have your ligne directrice, your guiding line, but you always add and change,” she says. “With a project like this, I love to see the before and after. When you put the period at the end, most of the time it looks better than what you imagined.”