1. A location on a floor where the building begins to "set back," or begins to have terraces
"So if you have a building that has the shape of a wedding cake, you have many 'penthouse apartments' there, which is great if you're a developer and you have five penthouses versus only one to sell," Gold said.
2. A unique layout from other units in the building
At 432 Park Avenue, New York City's tallest residential building, floors 91 through 96 are all referred to as penthouses because they have different layouts than the rest of the building and some are full floors, a representative for the developer told Business Insider.
3. More square footage than the top-floor penthouse
At the NOMA at 50 W. 30th St., one of Gold's listings at Corcoran, the top-floor unit is called "Penthouse 2," while the unit below was labelled "Penthouse 1" because it actually has more square footage overall and a different layout, as well as ample outdoor space and direct elevator access, Gold said.
4. Luxury amenities not found in other units
"Today, a penthouse can even be on the ground floor, if it's all tricked out and boasts luxury amenities not found in the building's other units," Cathie Ericson wrote for Realtor.com. "These might include upgraded appliances, increased security, fancier finishes, a larger balcony, oversize windows, a breathtaking view, a larger floor plan, and a private entrance."
Outdoor space is a covetable amenity on any floor of a building.
Sheila Trichter of Warburg Realty told Business Insider that apartments that sit a floor or two below the top-floor unit can be considered penthouses if they have terraces.
7. Expansive living areas with large windows
"... Very wide and very long great rooms and living rooms" with large windows are another desirable feature you can usually find in "penthouse-like" apartments, Lavrenov of Warburg Realty told Business Insider.
8. Fantastic views
At 15 Hudson Yards, an 88-story luxury residential tower just starting to welcome residents, penthouse units start on the 84th floor. Penthouse 84D has no outdoor space and it's four floors below the top-floor apartment. But with floor-to-ceiling windows in its corner great room, the unit offers "spectacular views of the Public Square and Gardens and Manhattan skyline," according to its StreetEasy listing.