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How can I legally and efficiently help my girlfriend build equity by helping with a mortgage?
There is no issue - and no question - if you get married.  The question is only relevant in the event that you go separate ways.  Should that happen, you imply that you would want to refund whatever amount your girlfriend has paid toward the mortgage. The solution, then, would seem to be to exempt her from any payments, as you will either give that money back to her (if you break up) or make her a co-owner of the condo (if you get married).  If you actually need her contributions to the monthly nut, you could give her a written agreement whereby you would refund her money (plus interest) at her discretion.