penthouse - A building on top of a flat roof, with walking space round it, such as an apartment of a lift machine room.
[Scott/Penguin Dict. of Building]

penthouse, pendice, pentice - A structure occupying usually less than half the roof area of a flat-roofed building, and used 1. to house equipment for elevator, ventilation or air conditioning, or other mechanical or electrical systems serving the building, or 2. to house one or more apartments ....
[Dict. of Architecture & Construction, McGHill]