Based on what? Are there a list of acceptable questions? I don't know about you, but "Is your wife pregnant" seems a bit intrusive to me. Almost discriminatory. There are laws against discrimination, so why is it within the law to ask such questions?

I accept that there are some apartments that have limitations based upon salary range. This is usually to allow lower paid people find acceptable accommodation. But these are usually co-opted properties that are run with that purpose in mind. And, let's be honest, those that are paid higher would probably not want to live there in the first place. And those that aren't Swiss stand little or no chance of getting on the waiting list in the first place (I know this from experience).

I'd be interested in any references that you have that quote the particular law related to this.