Not to be confused with common spaces open to all residents in apartment buildings, these are finished, underground rooms in ground-floor dwellings that the city does not consider habitable space.

Sometimes they are carved out of the cellars of old brownstones, sometimes they are excavated from deep in the ground of new condominiums, but they often come with a garden or an patio and add valuable square footage at a discount.

Because these spaces are not legally intended for sleeping, they can have bathrooms no larger than 5 feet by 5 feet, and cannot have a shower or a tub, but many developers — even those who have not put in a bathroom at all — install plumbing sufficient for a full bathroom in case a resident wants to put one in later. Mr. Maundrell recalled a buyer in a new development who put a shower into her half-bath, but was required to remove it when Buildings Department inspectors, examining the building to renew its temporary certificate of occupancy, discovered it.

“People play with fire when they do that,” Mr. Maundrell said. “I tell people, what you choose to do with the space is up to you, but what I’m selling you is a one-bedroom apartment that has a recreation space.”

In New York, square footage is square footage, whatever purpose it may ultimately serve — bedroom, home office, den, storage, even, yes, recreation. The Buildings Department calls these spaces, which are 50 percent or more below grade, accessory, on the theory that they are add-ons to the larger residential space upstairs, said Shahn Andersen, a developer who is active in converting Brooklyn brownstones to condos. Mr. Andersen carved a rec room out of half of the cellar for the garden apartment at 415 Clermont Avenue in Fort Greene, listed at $469,000 through Corcoran.