It’s common for real estate agents to refer to the top floor of any residential building as a penthouse. It may be even more common for real estate buyers and renters to sneer at referring to the top floor of a 2- or 3-story building as a penthouse.
We like Owen’s take on the topic:
It is at or near the top of a luxury building. It might be a duplex or all on one floor, with a terrace or with no outdoor space, but it is luxurious. It has views and it is unlike any other residence in the building. A penthouse has mystique.
We also like the description of the penthouse pictured above as “a rooftop 18-room bungalow.”