I guess it depends how old they are. If they're used to turning in at 10.30-ish, hearing you walking briskly across the room at 1am may not go down too well, especially if, as boxtops says, it's across wooden floors. Wooden floors should be banned in flats where there is someone living beneath them! They may also be really light sleepers, so will hear every little knock or squeak anyway. Not sure how opening the front door wakes up their son tho - you don't slam the door when you go in or out do you, so I'm not sure what that's all about? As boxtops says it's a shame they didn't knock "for a chat" as there's obviously ill feeling between you now which is always a shame.