Perhaps not a direct reply to your question, but keep in mind there are some strict rules on raising rents here. Perhaps good to check the local laws on renting, differs by Canton I think. Thus, even if the going rate for your apt. is say 4000 chf today, if previous rent was 1500 chf, you probably could not raise it to that, unless major renovations, etc.; as the increase cannot be more than X % or Y gross.