For the past 25 years, designer Jennifer Post has been crafting chic contemporary interiors for a client roster packed with luminaries from the worlds of entertainment, art and business. And for 20 of those years, she has called an apartment in the storied Upper West Side Apthorp building home. In the two decades Post has lived there, though, the personal home improvements she has fit into her busy schedule have been largely cosmetic—some paint here; new furnishings and art there. But that recently changed. “Finally, I had the time to do a gut renovation for myself and do full custom interiors that reflect my aesthetic: clean, crisp, soft yet elegant,” she says. “I’ve never lived in an apartment that I completely designed myself from the studs.”

Post dove all the way in, leaving no surface untouched during a year of nonstop construction. “I went down to the studs and original terra-cotta walls of the building,” she says. Tackling some layout changes, the designer moved the location of the master bathroom, turning the former site into a walk-in closet. In place of ornate architectural details—moldings, columns and replaces—Post chose a more pared-down look, with strategically placed recessed mirrors acting in their stead. She also completely rethought the materials palette; notably, the original parquet floors gave way to oversize slabs of Bianco Dolomiti marble, something the designer says she always imagined in her dream home.

The flooring, shot through with veins of gray and taupe, establishes an elegant base and points to Post’s overall design philosophy, which emphasizes a clean soft-white color palette to create warm, comfortable spaces. A Venetian plaster finish on the walls complements the marble and amplifies the effect. “It’s warmer than a simple painted wall—it has a soft, minimal movement and reflects light, which is wonderful,” she says. Plush silk rugs made by the designer’s custom workroom and diaphanous draperies also bring another layer of warmth, softness and ease.

“I love a soft yellow palette in general—it makes me happy—so I specifically wanted to add that in my study,” Post says. The hue appears in the room on an area rug, a ledge beneath the TV, parts of the built-in desk facing a window and the sunshine-bright pillows and throw accenting the deep cream-colored sofa.

When it comes to color, though, it is Post’s extensive contemporary art collection that makes the biggest splash—often it’s her artwork that inspires a room’s color scheme. She points to a large Jacqueline Humphries painting in shades of blue in her bedroom. “This is the first thing I see when I wake up, and it looks beautiful in the morning light,” she confides. It sets the tone for the blue-and-white bedding and a tailored armchair in pale blue with crisp white piping.

Besides the Humphries, Post’s ever-growing art collection includes a sculptural pair of dancing hares by Barry Flanagan, a Federico Herrero piece, two Robert Motherwells, a work by Russell Sharon, a sculpture by Boaz Vaadia and two others by Lynn Chadwick. All are newer acquisitions, and each was purchased following the designer’s simple criteria of choosing, she says, “things I know I will appreciate forever and that touch both my spiritual and intellectual sides.” As is the case with the Humphries—which always hangs in her bedroom—the larger pieces tend to stay in the same spots over time. Smaller pieces change positions or even lean against walls, like the Motherwell painting under a console in the foyer and photographs by Arnold Newman and André Villers on a bedroom shelf.

Complementing the designer’s art collection are furnishings that read sexy, minimal and sculptural. “I like classic contemporary furniture; it will always be in style,” Post says. “But truly, I only select extremely comfortable pieces—which, after my years doing this, I know how to choose very well.” For instance, not only does the white lacquer headboard in her bedroom define the space (“like a punctuation mark,” she says) and establish a cocoon-like atmosphere, but it also “creates a serene feeling without overpowering the clean lines of the space,” she adds. Likewise, a low-slung sectional and a chair in the living room strike a balance, managing to be stylish yet inviting. Custom built-ins read as architectural elements, offering functionality while keeping the rooms from seeming cluttered.

Though the reimagined bright spaces perfectly reflect her approach to projects, Post is quick to point out this isn’t a design lab or a showroom. Rather, it is a deeply personal refuge. “It’s my haven, and it allows me to have my lifestyle and be comfortable,” she says. “It’s all new, it’s white—which speaks to my core and my heart—and it showcases my art.”

—Lisa Bingham Dewart