When you talk about a building or some other fixed enclosed space, it is “the premises”. It is never “the premise” or “a premise”.
e.g. “She entered the premises.”
A railway compartment is never “premises”. A building or a hut is “premises”. 🙂
That’s all.
There is a singular “premise” but it refers to something that is a given, to support a point of view.
e.g. “She based her argument on the premise that the incident occurred before the law was changed.”