UH, OH! Here we go again, sorry to hear this. Yes, unfortunately they do have clout and plenty of it and if the authorities have there way they will try and screw as much money out of you as possible.

Officially, don't shoot the messenger, you need permission for any alteration to a casita. This means you are supposed to go to the adjuntament with your proposal. They will then check that the casita is listed on the deeds for the land ( some are not, even if they are old! ) you will be told to apply for architects plans!! As ridiculous as this sounds for a small casita. This will cost you an arm and a leg and you could be told no go anyway.

If they do approve your plans you will have to use a registered builder, can't DIY unless you are registered yourself. After the work is complete you will have to pay a tax to the adjuntament, which will be a percantage of the cost of the work.

Don't sit on this one, go to the adjuntamnet and try to find out where you stand.

Good luck