A 2,400-square-foot triplex penthouse residence is a world apart from New York’s notoriously cramped housing stock. But even rarefied living quarters have to contend with the city’s tight lots and dense urban fabric, which often eat up light and air. Just ask New York’s Turett Collaborative Architects (TCA), which worked with its client, a Manhattan-based businessman, to craft an inviting two-bedroom, three-bathroom apartment that maximized daylight from its northern and southern exposures, despite being closed in by an adjacent school and apartment tower.

A conventional master bath, isolated from the main living spaces, would obstruct natural light and decrease square footage, explains project manager James Saisakorn. Instead, TCA designed a vitrine-like 122-square-foot washroom, glassed in on three sides and cantilevered over the kitchen below. While TCA aimed to preserve daylight in the space, it needed an elegant solution for a potential privacy problem. With the flip of a switch, users can toggle the bathroom’s glass walls between opaque and transparent by activating a low-level electric current that runs between the panes. “It was an opportunity for us to create this dichotomy between public and private,” says Saisakorn.

The design team went with a depressed-tub scheme because that was the best way to preserve sightlines through the apartment. But finding a way to support the sunken tub—which is made up of two pieces of white synthetic surfacing and weighs in at 1 ton—was a particular challenge, says TCA founder and principal architect Wayne Turett. As a solution, TCA engineered a heavy-steel crossbeam system to brace the floor of the tub. “Luckily, we were working with an adventurous, design-minded client,” says Turett.

That spirit of adventure (and an undisclosed budget) led the design team and their client to Vals, Switzerland, where they visited thermal baths designed by the Pritzker Prize-winning Swiss architect Peter Zumthor and sourced quartzite for the apartment’s walls. Back in New York, contractors laid the quartzite according to an algorithm that accounted for the width and height of each slab. Larger blocks of quartzite were cantilevered from the wall to form a sink and a shelf. Above the basin, a long rectangular alcove in the wall provides space to store brushes, soaps, and other toiletries. “This was really an opportunity for us to work on design on the micro and macro levels,” says Saisakorn. “And I think this is a great example of how beautiful the meeting of materials can be.”