It is based on the application, type of building, type of AHUs, etc. Normally, it can be placed in a separate AHU room, or basement, or individual AHU room on each floor, or even outside as well. For example: In case of a large building, the capacity of each floor is very high and on each floor one dedicated AHU room is recommended. In case of a small building, based on the capacity of AHU, only a single AHU room may be sufficient for the entire building. In case of an industrial building, outdoor AHU is used without any dedicated room. In this case, double skin AHU is mandatory.