Premises is a curious word. The etymology as far as I understand is something plucked out of a legal document.¹
On the title deed of the document, the land or buildings are described at the start: they are the premise, which is to say the stated basis on which the rest of the document is based. So it might say
the property at 100 High Street, and surrounding land out, 100 yards north, and all attached and free standing buildings, including air rights, and right of free passage to the adjoining highway
(which is a mouthful). Then, the document proceeds,
these premises are rented to the Mr. John Smith, and John Smith is obliged to maintain these premises in good working order, and the lawn of the premises must be kept well
and so forth.
Premises is the plural of premise, a legal term meaning “the aforementioned thing”.² This was then plucked out of the contract to be used as a more general term to refer to the property and all the adjoining stuff. Eventually, the word took on a meaning of its own, and came to mean a building, or land or something similar.
So, I suppose the answer to your question is both yes and no. In the idiomatic usage referring to a building or land, it is always plural, but in its origin it can be singular.
