They seem to be advertising board for estate agents. The houses they're outside aren't for sale, though, and aren't necessarily even houses — there are four outside the allotments. They're used to publicise local events that seem to be sponsored by the estate agents. I've no idea whether the property owner gets any money for it, but suppose that they must do.
I expect that it's legal, but given how often the locals here feign indignance that some minor activity they don't like will "ruin the character of the village", I don't suppose these people will be happy (unless they're the ones with the boards outside their properties).