It is, by most measures, a standard-issue parking space, albeit one tucked under the Sterling Mason, a handsome building designed by the architect Morris Adjmi in TriBeCa. But the 12-by-24-foot spot below the lobby may well be the key to finally securing a buyer for the $15.5 million Penthouse C, the last unsold condominium in the 32-unit building.

Or at least that’s what the broker, Leonard Steinberg, president of the residential real estate brokerage firm Compass, believes.

Mind you, the penthouse, a four-bedroom, 4,986-square-foot duplex, has a lot going for it on its own, including interiors by Gachot Studios and French doors leading to a roomy terrace. But Mr. Steinberg, who was brought in by the building’s developer, Taconic Investment Partners, to market the holdout, is aware of the abundance of luxury condos on the market in TriBeCa. So he has decided to throw in the parking space as an added enticement, on the theory that buyers with the wherewithal to purchase the apartment might also own a vehicle.

“You can take the elevator down to your car,” said Mr. Steinberg, estimating that the parking spot is worth about $1 million, based on the sale price for a similar perk in a nearby building. After returning from a trip, “you can unpack comfortably and privately, without the hassle of holding up traffic on the street.”