an apartment located on the top floors of a building
"After years of saving, they finally moved into their dream penthouse overlooking the entire city skyline."
An outhouse or other structure (especially one with a sloping roof) attached to the outside wall of a building, sometimes as protection from the weather.
"The old farmhouse had a small wooden penthouse attached to its north side to shelter the woodpile from rain and snow."
To provide with a penthouse, shelter by means of a shed sloping from a wall, or anything similar.
"The old farmhouse was quickly penthouse after the heavy rains damaged the roofline."