Legal Question in Real Estate Law in California

First, you don't add someone to a deed. A deed is a single-use tool for transferring some ownership interest to another. You mean "added to the Title."

Next, you need to very clear that there is only a loose connection between being "on title" and being a party to the mortgage.

If X owns Blackacre and has borrowed $1 million from Shylock Mortgage Company on a note and first deed of trust, X's sale or gift of a half interest to Y does not make Y a co-borrower, and Y owes nothing to Shylock. However, X may have triggered a "due on sale" acceleration clause in the loan agreement with Shylock, and the entire loan balance might now be due and payable.

The moral of this part of the story is that no one should transfer an interest in encumbered property to someone else (except to his or her own trust), either for a fair price or as a gift, without getting approval in writing from their Shylock.

To continue: one of the side benefits of making interest payments on a mortgage is that the interest is tax deductible (usually). Maybe this is what you mean by "benefits of ownership." However, in order to deduct home mortgage payments, the person claiming the deduction must meet two qualifications. (1) He or she must actually make the payments. (2) He or she must be a person who was legally obliged (as a borrower/debtor) to make the payments. If you aren't a party to the laon, you can't take a deduction.

So, I guess the answer is that the borrower and original owner would continue to bear the legal burden of responsibility to Shylock for the payments, and would also be entitled to deduct the interest paid from his or her taxable income.

Is this what you wanted to know? In general, I take a dim view of adding anyone to title of property that isn't free and clear and where the person being added doesn't pay the fair value of the interest being received. Further, the pursposes and terms of the resulting co-ownership should be spelled out in detail in a signed writing. Think about what happens when you split up. Not gonna happen? Sure it will, unless you've discovered the secret of immortality.