"There is an office behind the building."
Translation:خَلْف اَلْبِناية مَكْتَب.
You don't need it for sentences of the kind they're teaching us in this lesson (so long as the subject is indefinite, as I understand it). Think of it as, "Behind the building is an office."
I guess the locative force of هناك is covered by the prepositional phrase as well? (i.e. having said "behind the building" we don't need to add "over there"...)