Utilize your financial knowledge, give your answer or opinion to the input question or subject . Answer format is not limited.

How can I legally and efficiently help my girlfriend build equity by helping with a mortgage?
Equity means having ownership, and I think that's a REALLY bad idea in the scenario that you described.  If you stay together, there's really no upside to either of you in this scheme.  If you break-up then you'll have a terrible mess, especially if the break-up goes badly.  If she's really building equity, you're going to be faced with several hard questions: If this went bad at the end, it might be worse than a divorce in some sense since at least in the divorce you have established law to sort out the issues.  You'll be on your own here without a formal contract. (Marriage being a special case of a contract for our purposes here.) If she wants to share costs (which seems perfectly fair) then agree to rent and a split on utilities.  If you really insist on going down the path that you described, I think that you'll need some sort of contract, which probably involves a lawyer.  Anything short of that could not be considered having equity at all and will be completely unenforceable in the event of a bad break-up. (There is some notion of a verbal contract, but that's very hard to prove and subject to misunderstanding and misremembering.) Aside from all of these potential problems in event of a break-up, you would probably also be violating the terms of your mortgage, if you have one. From the bank's perspective, you are selling the property that is the collateral for that loan, which you're almost surely not allowed to do.