At least in Berlin this doesn't seem to be true. While zoning laws are a bitch and exist for bribes, none of them prohibit intermixing offices with residential or retail with residential, and such buildings are very common.
Yeah true here in Spain it's very mixed too. It's better like that I think because this way the neighborhood doesn't become deserted at night and also we can go to a local restaurant for lunch.
The 'zoning' here is more building by building rather than neighborhood based.
But I mean the same kind of administrative issue holds back conversions here. Most Office buildings here would be ideal. They're not that big because here it's illegal to offer office space without plenty of daylight.
