Legal Question in Real Estate Law in California

As a co-owner, you have a right to occupy the building or any part thereof without payment of rent, so I'd say that the payments you are making are not truly rent nor is there a landlord-tenant relationship, unless perhaps you actually agreed that the sums were rent and that you'd become his tenant. The key here is that you and he could make a contract altering your rights and obligations, and it appears that you have done so; but now he wants to modify that contract, and he probably has no right to do so unilaterally. I'd advise seeing a real-estate attorney.

I agree with Mr. Whipple that as a co-owner with no obligation to pay rent, you can't turn yourself into a tenant by making payments toward the cost of ownership and calling it rent. The good news is that as a co-owner you have NO rent obligation, so other than a possibly enforceable co-ownership agreement that you would pay what you are paying, you can't be forced to pay anything more than you have been paying.

The other thing you and your co-owner need to realize is that what you are paying now is irrelevant to a buyer, for the reasons stated above. Buyers of multi-unit properties don't care what rent is currently being paid, only what they expect WILL be paid. Of course in a rent controlled building that is basically the same. But any buyer who knows what they are doing would treat your unit as being vacant upon close of escrow. It is not clear whether you intend to move out or not, but upon close of escrow you will have to either have negotiated a lease with the buyer or leave. So there is no relevance or reason to raise your rent now. The property will be priced on what you or a new tenant will pay the new owner.