"I am not formal at all," said designer Brock Forsblom’s client, who grew up on the Upper East Side. "I have a love affair with Los Angeles and want to get off the elevator and feel like I am in California." She appreciated Old Hollywood glam but also sought a bit of organic informality to counteract the chaos of living in New York. Her husband is a somewhat more formal sort, a suit-and-tie guy with refined taste. Forsblom imagined the home for the couple and their two young children as "Slim Aarons does Martinique," something casual yet glamorous. With lots of plants. (More on that later.)The original apartment was a classic six with over-the-top traditional elements wedged into a ’70s building with nine-foot-high ceilings. "It was full-on Missy Marie Park Avenue," Forsblom tells . "Hey, everyone has their own fantasy!" But the chunky crown moldings and chandelier medallions had to go. It was a (nearly) total gut renovation, the whole thing taking about a year to complete. Forsblom loved the challenge of designing everything from the new molding to the door paneling—as well as the furnishings. "As someone with a lot of traditionalist tendencies in my bones, it was fun to rip out a traditional interior and make it feel fresh."

In the library, Forsblom painted the wood a distinctive blue and kept everything else relatively simple, wanting to avoid an overabundance of fabrics and colors. The chairs, the clients’ own, were reupholstered in Lee Jofa mohair velvet. The 1970s Italian travertine and glass table is from Maison Gerard. The sofa, from Room & Board, wears a blue cotton velvet. Peacock-printed velvet throw pillows are from Studio Four NYC. The Paolo Buffa desk chair is from Gaspare Asaro. Sixties-era Italian brass sconces are from the Paris flea market. The smoked glass and chrome Italian desk is from Galerie Portuondo in Paris. The 1950s spoked chandelier with conical shades is from Rago Auctions; cabinet hardware from Rejuvenation; custom wall-to-wall carpet from ALT for Living; and Roman shades in fabric from Angela Brown.

A central wall was razed, but original parquet Versailles floors in these two rooms was left behind, albeit stripped and whitened. “Missy Marie sur la plage! And really, the parquet square is perhaps the little black dress of flooring,” the designer notes. Leaving some of the Park Avenue–ness intact in some places while completely abandoning it in others is what gives the place its considerable charm.

The new open arrangement is light and airy—in part due to the 15-foot-wide windows—but it did require some novel thinking. “The more you open up the plan, the more you restrict your material palette. Everything looks at everything.” Forsblom says it forced him to keep it tight, a process he found highly enjoyable. Once a few elements were set—oak floors, gray walls, white marble—it began to fall into place. “We were trying to create a really legible vocabulary for the architecture so that, as you move in the space, it feels really consistent.” Part of this scheme, involving relatively low ceilings, was the decision to have no full-length curtains.

To maintain this unity, every room got the same crown molding, every window got the same cabinet detailing below. “This idea of creating a really crisp, clean architecture was important, because you can be playful on top of that yet remain rigorous and fresh.” He also wanted to eschew the Park Avenue parade of styles that sometimes happens—an English country living room, a Georgian dining room, and a French library, what Forsblom refers to as a curio box of the last 300 years of design history.

In an NYC Park Avenue co-op apartment owned by a married pair of clients with dissimilar styles, designer Brock Forsblom embraced an open plan for the family room, kitchen, and breakfast room. “Suddenly everything feels sexy and young and livable,” Forsblom says. The blue mohair velvet Milo Baughman sofa is the clients’ own; the chandelier is contemporary Italian from the Paris flea market. Train table from Ducduc. Custom Moroccan wool rug from Rug & Kilim.

The kitchen floor is an update of a more traditional style, done in herringbone Gioia Venatino marble tiles from is painted in a custom gray.

and appliances are largely hidden behind the cabinetry. Plumbing fixtures are from . Hand-hammered cabinet hardware is from

Plants—and plenty of them—are an essential component of Forsblom’s design. It all starts with this self-irrigating living wall by Plant Wall Design in the breakfast room. The custom banquette wears a Brochier outdoor fabric from . Custom oak breakfast table. Chandelier from