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How to divide a mortgage and living area fairly?
I suggest that you first decide on what %'s of the home value you each have a legal claim to. Then split the mortgage using the same %'s. Then, if someone feels their % is slightly higher, they are compensated because they 'own' a correspondingly higher share of the house. Use the same %'s for downpayments (which may mean that an 'adjustment' payment might be required to bring your initial cash outlay from 70/30 into the %'s that you agree to). Tenant income gets split the same way. Utilities are a bit more difficult - as heating depends more on square feet, but water and hydro depend more on how many people are there. You can try to be really precise about working out the %'s, or just keep it simple by using the same %'s as the mortgage.