Legal Question in Real Estate Law in California

First, although you do not expressly say so, I assume from the given facts that you and your ex-partner own the residential property as joint tenants or tenants in common. For simplicity, I will refer to you as cotenants. Next, I'm assuming that you were not business partners nor registered domestic partners. I further assume you have no written agreement affecting your cotenancy. With these assumptions, here are some guiding principles:

Keep in mind that ownership percentages do not change, at least not directly, with the making of mortgage payments. That could happen if you were married and the property was originally someone's separate property, but that's a peculiarity of the community-property laws.

There is a statutory process for terminating an unhappy cotenancy. Either cotenant may sue for "partition" of the property. Originally, the court would then divide the ranch between the former cotenants, but nowadays partition is usually accomplished by sale of the home and division of the net proceeds (if any) between the former cotenants. The court can and will adjust the apportionment of the cash to reimburse a cotenant who has made payments in excess of his/her share for necessary expenses such as mortgage payments, property taxes, insurance and necessary repairs. Such an adjustment is not a recognition that an ownership percentage has changed; it is just a reimbursement for excess outlays.

However, under a principle called "purchase-money resulting trust," making a down payment that is disproportionate to the share of title received at the time of purchase can result in a cotenant receiving a larger equitable or beneficial interest than the legal interest shown on the recorded deed. For example, if you paid 100% of the down payment but got only a 50% interest "of record," you might have an equitable claim to be the 100% owner if you could also show that you didn't intend to make a gift of the other 50%. Who makes subsequent mortgage payments usually isn't a factor in determining whether there is a resulting trust.

Another concept is that all cotenants have the inherent right of possession of the entire property at all times. This doesn't mean that a cotenant out of possession can retake possession by breaking in, but he/she can apply for a court order allowing reentry. Related to this, neither cotenant has a right to receive rent from the other, but if either rents to a non-owner third party, the profits from the rental must be shared.

Finally, responsibility for mortgage payments is not dependent upon possession. The lender probably doesn't know and doesn't care who lives there. The persons who signed the promissory note are jointly and severally liable for each payment when due. Of course, with most mortgage situations, in the event of a default the lender must look to the collateral, i.e. must foreclose, rather than taking recourse against the borrower(s). Only if there is a deficiency after foreclosure can the lender possibly seek recourse against the borrowers personally, and then only in limited circumstances (e.g., the borrowers are protected if the lender used a trustee's sale rather than a court foreclosure, or if the loan was a purchase-money loan).

So, in sum, the changes that are occurring very likely will have no impact whatsoever on your ownership, but in the event of a partition action, the court could order some adjustment in the cash each of you receives based upon disproportionate past contributions to necessary expenses.