Go for it. There may be several contraventions. Buidling regulations
approval. Planning approval. Listed building approval. I might have
some sympathy with a homeowner making a mistake, a developer should
know better. If one person is taking the p***, others will follow and
you may soon live amongst stone cladding and upvc. If he didn't like
the rules, he shouldn't have bought the house, and left it for somebody
that does appreciate a conservation area.