Achieving the appropriate effect was not easy: any architect without a strong interest in geometry and construction might want to avoid this challenge. But Dean, aided by a can-do engineer and a willing contractor, seized the opportunity to create a complex roof structure containing the skylights. At first she thought it should be steel; her engineer sold her on lightweight concrete.

On top of that, Dean wanted the concrete to be contoured in certain areas to better bounce the light into the interior. “We turned to ruled surfaces, particularly those conceived by Felix Candela for inspiration,” she says, referring to the Mexican architect famous for his thin-shell structures. She adds that Bendy Ply formwork facilitated the curves. Two of the skylights bend accordingly, as does the long structural beam that shifts from a horizontal to vertical plane as it skims over the dining area and twists up to provide an opening for the stair.

Dean’s engineer and contractor, who had both worked with the firm before, were familiar with “our extensive design process,” says the architect. “They seemed confident that we knew how to build—certainly enough to get this to work.”

The resulting assemblage of concrete beams of various dimensions—some deep, some shallow, some upturned—now spans the original brick party walls. Suspended from the armature is the stainless-steel open-riser stair with wood treads.

If the construction sounds complex, it is. And the effect is surreal. Skylights of different shapes and sizes bring daylight into the central hall, extending from the main bedroom past the dining area to the living room, as well as to the two bathrooms and the kitchen. The apertures and smooth surfaces amplify the ambient quality of light as the eye travels up and out to glimpses of the rooftop and even the city’s towers looming above.

The choice of materials and finishes enhances the ineffable, dematerialized nature of the spaces: gray and white oak lines bedroom walls; elsewhere, sanded-aluminum panels give the storage walls in the hall, living, and dining areas a soft sheen. Ipé decking and precast-concrete pavers cover the roof terrace; concrete and white oak sheathe the floors inside. The muted tones of the setting highlight spare Midcentury Modern chairs by George Nelson and Hans Wegner and the cocoon-like suspended fireplace in the penthouse sitting room.

The structural acrobatics—which included raising the sixth-floor ceiling slightly but keeping the penthouse structure low, to conform with zoning restrictions on building heights—are impressive. The overall result makes the experience of moving through both levels a memorable reminder of the benefits of all forms of natural illumination in urban confines, no matter how daunting the effort.