Be sure to ask the locals how often they get away with that vs. how often foreigners get away with it (especially foreigners who've paid a 5000-euro deposit and explicitly promised not to do it, in exchange for a 1-euro purchase price)

Uh, no, that's a good way to get fined, at least in Portugal/Spain.

People will often circumvent rules when they can (e.g. add a second story without a permit, but avoid putting regular windows to claim it's just a storage attic), but if you want to make more profound changes, you better be prepared to fight the bureaucracy.