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How do I calculate ownership percentage for shared home ownership?
You and your husband are fronting all the money upfront. I'm guessing this will cost you around 67,000 once closing costs and fees are included. So obviously you would be hundred percent owners at the beginning. You'll then pay 31% of the mortgage and have your sister pay the remaining 69%. This puts your total investment at the end at 67k + 74.4k + 31% of interest accrued, and your sisters total investment at 165.6k+69% of interest accrued. If you hold the full length of the mortgage, your sister will have invested much more than you( assuming 30 year fixed rate, and 3.75%, she'd pay 116.6k in interest as opposed to your 49.6k) She will have spent 282.2k and y'all will have spent 191k. However if you sell early, your percentage could be much higher.  These calculations don't take into account the opportunity cost of fronting all the cash. It could be earning you more in the stock market or in a different investment property. Liability also could be an issue in the case of her not being able to pay. The bank can still come after you for the whole amount. Lastly and most importantly, this also doesn't include the fact that she will be living there and y'all will not. What kind of rent would she be paying to live in a similar home? If it is more than 1400, you will basically be subsidizing her living, as well as tying up funds, and increasing your risk exposure. If it is more than 1400, she shouldn't be any percent owner.