It is conditionally permitted.
I have obtained confirmation of this from the Zurich city police and from the Federal Data Protection and Information Commissioner,

Excerpt:
Video surveillance by private persons....private interest in ensuring that no burglary is committed during his absence. Video surveillance for the prevention and punishment of burglaries is therefore justified.....In the case of surveillance in an apartment building, it must not be possible to see who enters which flat or who operates which letterbox.

However, the administration can still prohibit it.