It’s every architect’s fantasy—getting carte blanche from a client. “It was excellent, and the first time for me,” Gus Wustemann says with evident glee, recalling how a couple contacted him after seeing his work in magazines, and offered complete creative license. The couple owned a 2,000-square-foot attic apartment in the historic quarter of Lucerne, Switzerland, and wanted it not just renovated, but transformed.

“The place was a black box—practically no context whatsoever,” Wustemann says of the loft’s original condition. The architect, who has offices in Zurich and Barcelona, specializes in sleek, airy residential spaces.

The clients, a food business manager and his wife, delivered a brief of only two points: a light-filled loft and a better way to access their roof terrace (to replace the extremely ordinary existing stair). With these parameters set, they let Wustemann get to work. No histrionics, no difficult contractors, no budget-related delays. Half a year and $190,000 later, the apartment was move-in ready.

Wustemann—a Swiss native and longtime mountaineer—took inspiration from the glacier as a metaphor for the stair to the terrace and the loft’s entire “landscape.” The motif would connect not only the apartment to its roof deck, but also this most Modern interior to the nearby alpine scenery. “Getting to the light, and to the terrace, is a little like getting to the summit,” he explains, envisioning the loft’s components as part of the ascent through a glacial landscape.

To set the tone for that narrative, Wustemann coated the floors in white polyurethane, creating a “frozen lake” beneath his glacier. Offsetting the coolness of the floors, warm, honey-brown, unvarnished oriented strand board (OSB) forms the bathroom and service cores. For the built-in furniture and cabinetry, the architect chose the same inexpensive material, which offers a desirable sustainability through its reconstituted content. Elsewhere in the loft, OSB reappears in two other finishes—slick, lacquered white for the glacier stair, kitchen, and bedroom; and whitewashed for the existing structural walls—calling to mind the transformative states of such natural substances as stone eroding into sand or water chilling into ice.

Most of the loft, apart from the cores, remains open. Though the place, roughly trapezoidal in plan, appears free-form, the glacier stair is its centerpiece, immediately visible on entry. Here, Wustemann created a lacquered sculptural wall that rises with a meandering path of asymmetrically placed steps. Niches articulate the glacial cascade, casting an animated play of shadows, while offering nooks for object display or storage. Also on the stair, friends can gather and chat or watch dinner preparation.