You can go ahead and build your extension/modification without planning permission and the townhall will eventually fine you. The fine is a percentage of the added value to the propoerty. The modifications will never be legal but once you've paid the fine you retain this paperwork with the title deeds as proof that you've paid the fine and that should be the end of it
Several owners at Alta Entenas (including me) were advised to do this, the town hall eventually imposed a fine which some owners are now contesting. I challenged the townhall on the added value and came to a mutal agreement and paid the fine. This has been a financial benefit to the town hall providing an additional income.
I'm not suggesting you should go ahead with this option but it has been common practice.