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How to divide a mortgage and living area fairly?
In my opinion, since she will live in one apartment, as will you and your husband, the simplest method is to divide the ratio exactly the same as the area for your living space. If it's 40/60, she puts 40% down, you put 60%. And you split expenses the same.  The tenant income can be applied to the house expenses, as it's no different than giving her 40% and you keep 60%.  No matter how well you get along, it's easy for someone to feel a split of expenses isn't fair unless it's discussed and agreed up front.