But it's absolutely unheard of
it can't be.. or why would it be so..
What are the documents requested from foreigners to rent a flat long term?
depends.. ask prospective landlords
It's not a bad idea to also ask if the owner has permission from the administration to rent the flat
talk about unusual and most likely a fat chance of getting such info from a prospective landlord.. hmm.. i know, that i would rather frown upon such request and most likely wouldn't oblige.. but you are right, not a bad idea at all.