Legal Question in Real Estate Law in California

"Could try" is the operative word here. When a couple is married, a separately-owned property can acquire a community interest through the making of mortgage principal payments because the earnings of either spouse during marriage is community property. If you're not married, there is no legal principle that brings about an ownership transfer, at least not without a lot of creative pleading to establish promises you didn't make and that the judge or jury believes, nevertheless, that you did make. So, while a written agreement is technically unnecessary, having one is in reality a darn good idea. It should include reference to the amount (percentages or flat amounts) of cost-sharing by category and characterize his right to co-occupy the property as a revocable license and not a tenancy.

I agree with Mr. Whipple as to the issue of making payments towards the mortgage. I write separately to address the second issue, which is payment of rent. The payment of rent creates a tenancy, and would cause the other person to become a month to month tenant. That meant when you wanted them out, you would have to follow the law for evictions. It may be easier to have a written agreement that clarifies that it is not rent, but rather contributions towards food and utilities.