How to calculate ownership for property with a partner
i would recommend that you establish a landlord/tenant relationship instead of joint ownership (ie 100% ownership stake for one of you vs 0% for the other). it is much cleaner and simpler.  basically, one of you can propose a monthly rent amount and the other one can chose to be either renter or landlord. alternatively, you can both write down a secret rental price offer assuming you are the landlord, then pick the landlord who wrote down the smaller rental price.  if neither of you can afford the down payment, then you can consider the renter's contribution an unsecured loan (at an agreed interest rate and payment schedule). if you must have both names on the financing, then i would recommend you sell the property (or refinance under a single name) as quickly as possible when the relationship ends (if not before), pay the renter back any remaining balance on the loan and leave the landlord with the resulting equity (or debt).  in any case, if you expect the unsecured loan to outlive your relationship, then you are either buying a house you can't afford, or partnering on it with someone you shouldn't.