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How can I legally and efficiently help my girlfriend build equity by helping with a mortgage?
all the other answers are spot on, but look at it this way. really all you mean when you say "building equity" is "accumulating wealth".  if that is the goal, then having her invest the money in a brokerage (e.g. ira) account makes a lot more sense.  if you can't afford the apartment without her, then you can't afford to pay out her portion of the equity in the future.  which means she is not building equity, you are just borrowing money from her.  the safest and simplest thing for you to do is to agree on a number that does not include "equity". to be really safe, you might want to both sign something in writing that says she will never have an equity stake unless you agree to it in writing.  it doesn't have to be anything fancy. in fact, the shorter the better. i am thinking about 3 sentences should do the trick. if you feel you absolutely have to borrow money from her on a monthly basis to afford your mortgage, then i recommend you make it an unsecured loan.  just be sure to specify the interest rate (even if it is zero), and the repayment terms (and ideally, late payment penalties). again, nothing fancy, 10 sentences maybe.  e.g. "john doe will borrow x$ per month, until jane doe vacates the apartment. after such time, john doe will begin repaying the loan at y$ per month...." that said, borrowing money from friends and family almost never turns out well. at the very least, you need to save up a few months of rent so that if you do break up, you have time to find another roommate. disclaimer: i do not have any state-issued professional licenses.