Following a pilot boutique designed by Belgian interior architect Lionel Jadot, which opened in 2013 in Aspen, Colorado, where Ullens has a home with husband Guy, the fashion entrepreneur set her sights on Paris. When Ullens approached OMA's Hong Kong office—with whom her husband had worked previously—her only remit to the architects was to create a flagship boutique appropriate to her brand. Neither Ullens nor her husband are put off by ambitious projects. They had earlier commissioned Jean-Michel Wilmotte and Qingyun Ma to transform a 1950s industrial building in Beijing into the Ullens Center for Contemporary Art—opened in 2007—to house their personal collection of more than 1,000 pieces.

OMA visited several potential locations in the Golden Triangle with Ullens. Despite being on a quiet side street, a building on the rue de Marignan proved best—even though it was subdivided into three separate spaces by 2-foot-thick existing walls that could not be altered. “If you were to build a new store, you would never do it this way, but it provided an opportunity to create layers within the space,” explains OMA managing partner David Gianotten.

Ullens highlighted eight key attributes she wanted her boutique to convey: comfort, exclusivity, care, femininity, art, voyage, detail, and senses. “These qualities specified the vibe, but not the materiality of the space,” says Gianotten. It was the subdued luxury of the clothing itself that gave rise to an architectural vocabulary he describes as simultaneously “minimalistic and tactile.”

“We chose to work with materials that transition over the course of the day,” explains Gianotten. Onyx, cream-colored leather, terrazzo and carpet, brass and teak create an airy and neutral yet sensual backdrop for Ullens's set pieces, the fashion. As one mounts three steps to enter the boutique, a glazed door automatically glides silently sideways, opening into an intimate foyer. Visitors are greeted by two Ullens-draped mannequins that stand juxtaposed against a full-height onyx wall.

Composed of ¾-inch-thick sheets, which range in size from 2 to 3 feet square, the onyx is wet-fixed to the wall, which creates a translucent effect. Immaculate attention to detail pervades the store. Faux leather is folded around hard foam on a plywood backing and then top-stitched, using patterns based on Haussmannian molded wall details, with blind stitching that creates a rectangular paneled effect. A ¼-inch-thick copper profile delineates the joint between the carpet and the terrazzo floor. Other walls are clad in acid-etched glass back-painted white. Woodwork throughout is teak veneer in 2-foot-wide panels, prefabricated in Italy. The architects brought life to the existing thick walls by cladding the framed openings in natural brass; a short passageway brings one face-to-face with a bold wall-hung Anish Kapoor—part of the Ullenses' personal art collection. The passage leads to cocoonlike fitting rooms lined in curved teak. The ensemble exudes an atmosphere of understated exclusivity.

Although early spring sales were slow at the rue de Marignan, OMA is currently at work on a second store in an undis-closed location, as well as a prototype “shop-in-shop” design, which Ullens plans to roll out in department stores. A popup shop on London's Walton Street is testing the UK market.

Gianotten observes that Ullens's personality permeates the business. Telltale signs are the boutique's logo, which is subtly embossed on brass hooks and woven into the carpet. “The project was very special. Clients who are so personally involved are rare,” he adds. There is no disputing the visual appeal of OMA's elegant boutique for Maison Ullens. Whether that will translate into lucrative sales for the nascent Belgian clothing brand remains to be seen.