We have not long bought here in the campo & were advised by the local & very lovely David Pique not to think about extending & used the same argument that hhmunroe used, would you do it in the Lake District?
The owner (who was Spanish) who sold to us had to pay back taxes etc on a brick porch on the front of the house (abt 2sqm), & what they also called a porch which is an area covered with a tin roof, it's considered a permenant structure so illegal! He was lucky & didn't have to tear down!
Buy somewhere that has the footprint you need & renovate!