I would be inclined to agree with Huw, I know of a few Catalans who have been evicted from their houses which they bought from "reputable" estate agents and were approved for mortgages by spanish banks.  The problem arose when certain areas were found to be zoned differently.  I think if proper due diligence was followed and searches conducted they might not have fallen foul of the authorities because in at least one case I knew that the group of houses were due for demolition - it was a matter of public record.  If the right questions are not asked, you won't be in possession of the full facts.  I don't think it's fair to say that the aim of the local people and authorities is to fleece foreigners while ignoring local transgressions. 

I know of a few people (both natives and foreigners) who chose to ignore the writing on the wall.  In matters relating to property and escrituras, a casa is a casa and an almacen is simply an almacen.  Pumping hard earned cash into what is a shed and transforming it into a perfectly habitable home doesn't change what it is until the escritura reflects this.  The same goes in Ireland and the UK I imagine.  For example, my local councellor might come out and look at a proposed site and nod and wink in all the right ways and declare that he can see no problem with the proposed project but until I apply for and receive full planning permission to construct the dwelling it can never be legally recognised as such. Indeed in Ireland houses have been demolished because they didn't have planning permission or were not constructed in line with the permission given.

Introducing retrospective legislation though is a nightmare I admit.