I've just moved into a first floor flat in a semi-detached 2-storey 1930s/40s house, and realise there is little to no soundproofing between me and the ground floor flat below. I therefore want to soundproof my floor - probably involving taking up the floorboards and putting in some mineral wool/acoustic quilt. My dad raised the question of the law - i.e. in doing this, would I need to seek the permission of the flat-owner below me? I own my flat but it's leasehold, and I believe it's the same with the person below but I'm not sure.