When we moved in our neighbours told us he had moved in with his girlfriend years ago and not sold the house, on the basis that the relationship might not work out and he'd need somewhere to move back to.
Has it been long enough to say his relationship worked out and I can knock the wall through to steal his bedroom to expand my own? I'd like more space and with my desk and wardrobe this room is pretty cramped.
Could you knock it through and then build a new wall that looks identical but is on rails or wheels or something so if he comes back you can quickly put the wall back to normal before he notices? Maybe jam the door so you have more time.