What is the name of this thing like a building on top of a roof?

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?

Solution 1:

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. From Collins Cobuild:

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

Solution 2:

The Scottish Building regulations manual calls this a

rooftop plant room.

Scottish Building Regulations

Solution 3:

The term mechanical penthouse (or MPH) is common in North America, especially on taller buildings.

Solution 4:

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):

bulkhead

4. Building Trades.

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

I'm thinking 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.