It's like a building on top of a roof. I don’t know how to describe it too well, but you can walk out of it and be on the actual building's roof. This building thing is always on a roof, and can have AC coolers on top. Sadly I’m ill so the name is lost to me. Help?

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    Elevator equipment room. – Phil Sweet Dec 29 '17 at 1:35
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    Or, more broadly, mechanical room... – Rob_Ster Dec 29 '17 at 1:58

Although the most common use of the word penthouse is the biggest, most luxurious apartment at the top of a tall building, in British English at least, it is also used to describe the building your photo:

penthouse in British (ˈpɛntˌhaʊs ) noun

  1. a flat or maisonette built onto the top floor or roof of a block of flats
  2. a construction on the roof of a building, esp one used to house machinery
  3. a shed built against a building, esp one that has a sloping roof
  4. real tennis the roofed corridor that runs along three sides of the court
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  • Also: penthouse in American: 3. a an apartment or other houselike structure built on the roof of a building (ibid.). – Kris Dec 29 '17 at 10:43

If it were smaller and only intended to be a weatherproof enclosure for the top of the stairwell, you might call it a bulkhead:

Quoting dictionary.com (Random House dictionary):


  1. Building Trades.

    b. boxlike structure, as on a roof, covering a stairwell or other opening.

I'm think the specific kind of structure you see there is somewhere between this word and penthouse, as advised in another answer, so I'm somewhat at a loss to suggest one over the other.

It might be better described by its in-context use, e.g. utility room or roof access structure.

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The Scottish Building regulations manual calls this a

rooftop plant room.

Scottish Building Regulations

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I'm quite sure you are referring to the attic.

An attic is defined as a space or room inside or partly inside the roof of a building.

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    I don't think the construction shown in the photo would be called an attic. An attic is under the pitched roof of a traditional building. The picture shows a modern flat-roofed building. – Kate Bunting Dec 29 '17 at 9:54
  • Note the "inside or partly inside the roof" -- this one is on the roof. It's a "penthouse". – Kris Dec 29 '17 at 10:39
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    This is definitely not an attic, either by definition or apparent use. An attic is usually either a storage area or an improvised living area whose "walls" are defined more by the lines of the roof than those of the walls below the level of the attic. – Aiken Drum Dec 29 '17 at 13:35

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