While the advice you have received so far is very good, A BMW message board is probably the last place you should be getting advice from on this very complicated matter. Go find a good mortgage banker, accountant, financial advisor and lawyer (that's 4 separate people) and listen to everything they have to say.

And don't buy a house with your girlfriend.

EVER.

Good luck have fun.

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You have your way. I have my way. As for the right way, the correct way, and the only way, it does not exist.