I believe for hdb units - the costs is to be shared 50/50 with the neighbor upstairs / downstairs. For condo, I understand it is to be settled between relevant neighbors. My relative’s neighbor upstairs settled the full costs of repairing / renovating their bathroom fortunately. I think it would have been a hassle if have to negotiate cost sharing and amount cos the price of renovating the bathroom can vary very much depending on materials / contractor used. My relative settled the cost of replacing the damaged false ceiling.