Landmark status matters

Owners of rooftop rights can expand if the building is underbuilt, meaning its zoning allows more floor area than it currently occupies. All it takes is a building permit. However, if the building is landmarked or in a landmark district, involvement of the Landmarks Preservation Commission makes it a more uncertain process.

"At my project at 2123 S. William St. in the historic Stone Street district, we negotiated with Landmarks for a year to add a penthouse," Korves said. "They wanted it to look like a traditional superintendent's penthouse."

Eberhart Brothers, a company that owns and manages more than 60 buildings on the Upper East and Upper West sides, has built about 52 rooftop units, marketed as "supershares," over the past 12 years. Construction costs are "on a par with new high-rise construction in Manhattan," said Jason Enters, director of design and construction at Eberhart. That could be as much as $400 to $500 a square foot. It's worth it, Enters said.

"The excess air rights on these walk-up buildings we own are essentially free land in New York City," he said.

All it's cracked up to be

Matt Blackman, a hedge fund employee, splits his $4,650 a month rent for a 1,200-square-foot "supershare" on a five-floor walk-up at 229 E. 89th St. with two roommates. "I'm a raving fan of these things," he said. "It's a home run for single people. It makes a fun city that much more fun."

Although it takes up to eight months for Eberhart to build a unit, creation of rooftop homes may be getting easier. Besides a firm called Loft Cube Co. that will literally drop a home onto your rooftop, a business called Top Penthouse designs custom modular penthouses. Co-owner Myriam Castillo did a study of the Upper West Side from 72nd to 96th streets and found almost 1 million square feet of unused floor area at about 300 different sites.

Top Penthouse built its first unit last year, a 400-square-foot rooftop studio designed by architect Charles Mallea on a walk-up at 114 St. Marks Place. "It was an incredibly cool project," studio owner Laura Harrison said. "Just the fact that you could build something within the space of two months, instead of having workmen in and out of the house for ages, appealed to me. "Though construction costs were not cheap at around $200 a square foot, the project took about five months from conception to occupancy, and only one day to hoist the five modules to the roof with a crane, Castillo said. Expediency is highly marketable in rooftop construction. Often, a typical unit can take a year or more. "These projects always tend to start out as wrecks," said Jane Siris of Siris/Coombs Architects, who designed Topping's penthouse and is a veteran of rooftop design. "It's always very tricky business. "The ceilings of existing apartments may have to be ripped up, there often is leakage, construction materials and equipment have to be hoisted, weight issues must be considered and there are endless problems with piping, Siris said. "There are a lot of structural issues - that's what makes these so interesting," she said. The resulting structures can be quirky. Topping's well-appointed apartment has a long narrow kitchen to work around the elevators and fire stairway. A rooftop neighbor, Susan Ross Green, created a whimsical mini-terrace instead of enclosing an ancient leak in her 54-year-old building.

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Modular Construction in New York