Is it proper to say the demised premises is or the demised premises are?

In a Lease agreement for a Building being constructed, it may say:
'said premises and the building and other improvements thereon, upon completion of construction, are hereinafter called the "demised premises".'

Etymology and usage: 'Premises' is from Medieval Latin praemissa, a plural Word, meaning the "things mentioned before" and was used in legal documents. In Middle English, premises was defined as being "that property, collectively, which is specified in the beginning of a legal document and which is conveyed, as by grant, …". In due course premises was also legally extended to mean "a house or building with its grounds or appurtenances," a usage which was recorded before 1730.
In a general sense today, premises means a piece of land together with the building(s) on it, or even just a building with all its parts. Therefore the Word 'premises' is always plural when used in this way.