This should be decided by locals, but as general critera we should follow the verifiability Wikipedian principle.
Those buildings may be included in some official register of whatever
type, national, regional or local. Those registers probably have some
kind of ID. This ID will be unique at his level, regional or local. If
you want to make them also unique at national level, then you must
prefix it with something like "register + id"
I have a question:
Can we accept images in the contest on items that do not have a unique ID?
In Sweden we have many buildings that are protected at the local level, because of its historical value, but are not necessary included in the National Heritage Board's records. We also have protected ships, and other types of monuments (such as statues, and ancient monuments). We also have Fornminnesregistret
(National Ancient Remains register) containing more than 1.7 million entries.