Offer your thoughts or opinion on the input financial query or topic using your financial background.

How can I legally and efficiently help my girlfriend build equity by helping with a mortgage?
I'm glad that you feel like being fair and equitable to your party. Other answerers are, of course, correct that being fair and equitable to your girlfriend is not in your best interests but that's not what you're trying to do here and I commend you for it. There is nothing that stops you drawing up a simple legal contract giving your girlfriend a share of the value of your house in return for her payments. Just get it signed and witnessed and checked over by a legal representative. You can include reasonable terms for the money to be paid back if you separate - perhaps when you sell the property or within two years of the breakup - that don't put you in immediate danger of losing the property. Just make clear that this contract is between you and her for a sum of money linked to the value of your house; it does not establish any legal claim on your house itself. A reasonable level for her to claim the property would be one half of the change in equity between when you start joint paying and when you separate - should that happen.