Neighbour downstairs planning on knocking down walls, what permission does he need?

casper_gutman

I haven't replied, because I have nothing to add to the advice you've already been given.

Yes, you could just let your neighbour crack on and cross your fingers hoping the builder is right and everything's okay. It's quite possible, likely even, that the builder is correct and there won't be serious problems. But don't think for a moment that this work is risk-free. If you want to minimise the risk, you should politely insist your neighbour does things properly. Your home is literally at risk here.