Definition of premises (1): land, and all the built structures on it, especially when considered as a single place; (plural only; not used in singular form) The subject of a conveyance or deed
Definition of premises (2): premises n. 1) in real estate, land and the improvements on it, a building, store, shop, apartment, or other designated structure. The exact premises may be important in determining if an outbuilding (shed, cabana, detached garage) is insured or whether a person accused of burglary has actually entered a structure.
The term premises (as used in Australia anyway) relates to the use of the land and building/s. It's premises if it is within one building. If it is one use within three buildings, it's still premises!