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Is this room already adjoining his? I’d be wary of capitulating- can you hear him already? He can’t really be asking you to change the original part of the house just because you’re extending. Sadly, it’s often those who have lived there the longest who seem to think they control the neighbourhood. But if you are doing anything in that room, you could install rockwool sound insulation between the joists which is always a good thing anyway (and won’t interfere with the features). I’d also be prepared for him to demand pw surveyors etc as there won’t be any appeasing him no matter how considerate you are