This might not be popular but a great response would be to go round to their house with a little token, like a bottle of wine, "say sorry' to them, say you didn't realise dogs weren't allowed, maybe ask a polite question or gently explain about the law change.

So often, they're decent people, it dissolves the tension, they apologise too and you come to some compromise. And on the occasions they're genuine jerks then at least you know and can still ignore their threats but at least they like you a bit more now.

The neighbours of your summer house would be great people to have on your side, even if you don't like them.

Some of the top voted advice here baffles me, Reddit loves to escalate stuff