"He’s good at puzzles,” says Wonbo Woo of his father, architect Kyu Sung Woo. That makes Wonbo a fortunate son, as the spatial challenge posed by the 30-year-old ABC News producer’s loft—which Woo père had offered, as the ultimate in housewarming gifts, to redesign—was puzzling indeed.

The apartment, located in a converted Union Square hat factory, captured the younger man’s attention with its eccentric qualities—it’s 50 feet in length, but only 12 and a half feet wide—and the fact that the undulating vaulted ceiling was comfortingly familiar. “My father designed the house I grew up in in Cambridge [Massachusetts], and it had a vaulted ceiling in my bedroom,” Wonbo recalls. “I’m not sure how conscious it was, but the loft actually did remind me of home.”

The developer renovation that had rendered the not-quite-700-square-foot space a tangled web of unrelated rooms, however, inspired no such warm feelings. Entering, Woo encountered a bathroom and kitchen, the areas above them completely boxed in—“they didn’t know what to do with the space, so they just dropped the ceiling,” he says—followed by an enormous sleeping loft that sat atop a five-foot-high storage enclosure. “That’s typical in Manhattan loft construction, where you don’t have quite enough for two full floors,” explains associate architect Choon Choi, indicating the ceiling, which rises to just over 12 feet at its peak. “They line up all the elements side by side against one wall, and block off the spaces above and below.”

Whatever the rationale, the overall effect was grim. “It was cluttered and tight, not spacious at all,” Choi recalls. “And very dark,” adds Woo. “The back of the sleeping loft was a flat wall that stopped the light.”

The goal of the renovation, says Choi, was a balance between density and porosity: “We set out to maximize the space—to make full use of every cubic inch of this volume—without blocking anything out.” The client, too, sought an interlinking of opposites, though of a different kind. “I’ve inherited my father’s aesthetic,” Wonbo observes, referring to the elder Woo’s modernist vocabulary. “But if I have a complaint about modern design, it’s that it’s sometimes not”—he hesitates, then utters the C word—“cozy.” Recalling the home in which his family lived prior to his father’s creation, an 1870s Cambridge residence, Wonbo says, pointedly, “I was glad to have had the experience of living in a cozy place.” Thus, whatever other feats the design may accomplish, an infusion of this intangible element remained essential.

To determine just how much unseen space they had to work with, the architects cut a small hole into the wall abutting the sleeping loft, peered in, and made a startling discovery: Not only was the area above the kitchen and bathroom completely unobstructed, they could see all the way into the identical space in the apartment across the hallway. “There was no closure in between,” Choi says, adding, with a grin, “during construction, it was very tempting to just kind of build into that loft.”

Though he resisted this secret annexation, architect Woo recognized that the drawback that defeated the developers—the not-quite-high-enough ceiling—could be overcome using the skill cited by his son: a knack for puzzles. Having decided to site the new loft bedroom directly above the kitchen, the architects met the challenge of stacking two rooms, each with a seven-foot ceiling height, in only 12 feet of vertical space by creating two interlocking puzzle pieces: The mattress in the bedroom sits directly atop the ultra-thin kitchen ceiling (which enables a full-height space downstairs), and the floor area around the mattress is two feet lower than the platform on which it sits (thereby creating a full-height circulation area up above).

The puzzle’s success, of course, is built on the user’s expectations. “When you walk into a bedroom, most of it is taken up by the bed, which is usually two feet higher than the floor,” Choi explains. Such is the case here—the difference being that the “platform bed” that supports Wonbo’s mattress is actually hollow, its empty interior space filled by the upper part of the kitchen. As Choi puts it, “Rather than putting a bed on top of the floor, we raised the floor to form the bed.”

The architects also made productive use of the volume separating kitchen from hallway, inserting the refrigerator into one side and the loft’s principal closet in the other. And they revealed their “trick” by leaving the edge of the upstairs floor exposed, an elegant architectural gesture that’s practical as well. “If there’s someone up top, you can hand something to them more easily,” Choi explains. (A panel of tempered glass protects Wonbo from accidentally tumbling from bed into the kitchen.)

Most people would be satisfied extracting one decent-sized bedroom from such minimal square footage. But, as Wonbo puts it, “I was hoping to have a second bed, so I wouldn’t have to give mine to my mother every time she came to visit.” Although some psychotherapist has surely been deprived of a client as a result, Woo satisfied his son’s request by slipping a second sleeping platform above the bathroom. The space is tighter than the “master suite” (and low-ceilinged, as the interlocking-puzzle strategy was thwarted by spatial limitations), but it remains an effective short-term accommodation.

The design team minimized renovation costs by purchasing nearly all the hardware and appliances from catalogues and websites; this includes a loft stair, made to measure by a company called Lapeyre Stair, that resembles an exercise machine but is in fact a space-saving alternative to a ladder. They also sharply limited their palette. “If you take out the lighting, hardware, stair, and appliances, there’s little left except drywall,” says Choi. “We have just two materials,” he jokes, “maple and paint.”

Yet the exceptional thoughtfulness of the design—the way it not only interlocks but overlaps rooms, compresses and releases space, withholds and reveals views, contrasts the angular and the planar, and preserves unbroken the long, flowing expanse of ceiling—makes the loft seem more complex, and much bigger, than it is.

“I’ve lived in New York ten years, so I’m definitely used to small spaces,” says Wonbo. “This feels palatial. It’s almost like having a two-bedroom.” Yet he believes that an essential pleasure of living small has been preserved. “The nice thing about a small space is that it’s intimate, and there’s still quite a bit of that remaining here,” he says. “There’s a very loft-feeling living room, and there is a”—yes—“cozy feeling upstairs.”

It’s possible that some of that coziness derives from the fact that many of the design elements, including the Lapeyre stair, appear in the Woo family’s Cambridge house. “It’s kind of shocking,” Wonbo admits. “It’s not like I went home again—I feel like home came here.” Which, he suggests, is not entirely a bad thing. “There’s definitely a feeling of my dad in this place. And of our family.”