Most of the millions of people who walk, bike, hail cabs, wait in long deli lines, ride double-decker tourist buses and otherwise go about their usual business in New York City every day never see vast sections of the metropolis that are hidden from the casual observer?s eye. You could explore every last corner of Manhattan, gazing up at all of the architecture, without realizing there?s essentially a city on top of the city that can only be taken in as a whole via plane or helicopter.
New York City maintains stringent building regulations to preserve the facades of its historic landmark buildings, requiring any rooftop additions to be set back far enough to be invisible from street level. This rule applies to all the luxurious penthouse gardens, terraces, swimming pools and greenhouses that have sprouted up on the city?s rooftops for over a century, but as developers look to add more square footage to the densely populated urban center, they?re increasingly creating an exclusive ?stealth? version of the city, full of modern structures.
A jagged loft addition recently completed by the firm WORKac serves as a dramatic example of what might be too far above your head to notice. Named ?The Stealth Building,? the hidden structure adds a three-story penthouse to the roof of one of Tribeca?s oldest cast iron facades. The addition is hidden from view not only due to its height, but also by the flat, windowless walls of adjacent structures.

An abandoned elevator bulkhead and t...