Planning enforcement for domestic development in England is definitely 4 years, I agree with the parent comment.
There was that one guy who covered his castle in hay bales for just over 4 years before revealing it and they took enforcement action successfully, but that's a bit different. I'm guessing that's why you said "became obvious that building works had been completed" but actually the rule is simply completion date but the courts decided that this guy was taking the piss so much that he still violated the intended meaning of the law. Your odd agricultural shed example may well fall foul of the same thing but I don't see what that has to do with the parent comment.
It's 10 years (not 12) in some cases but they wouldn't concern most domestic developments.
Any insurance of the type referred to by the parent comment is 100% definitely a swindle.