"I have no idea if it's an urban legend, but I have heard a few times over the years some people say that "if you can build it in a day, it doesn't need planning permission"
@LurkingHusband - I believe there used to be a form of squatters rights, on common land where you had to build a dwelling with a roof and working chimney/fireplace in a day, and then you stood at the four corners of the building and threw your axe as far as you could, and those four points marked the four corners of your land. You could then live there and work the land.
I was told this by my dad, who was a keen amateur local historian - it was the explanation for the little enclaves in the hill near us - all abandoned and gone to rack and ruin, but you could see there had been a dwelling and enclosed land there.