Cal. Civil Code § 1807.1

(a) The holder of a retail installment contract may, upon agreement with the buyer, extend the scheduled due date or defer the scheduled payment of all or of any part of any installment or installments payable thereunder. No charge shall be made for any such extension or deferment unless the agreement for such extension or deferment is in writing and signed by the parties thereto. (b) Where the contract includes a finance charge determined on the precomputed basis, the holder may charge and contract for the payment of an extension or deferral agreement charge by the buyer and collect and receive the same, but such charge may not exceed an amount equal to 1 percent per month simple interest on the amount of the installment or installments, or part thereof, extended or deferred for the period of extension or deferral. Such period shall not exceed the period from the date when such extended or deferred installment or installments, or part thereof, would have been payable in the absence of such extension or deferral, to the date when such installment or installments, or part thereof, are made payable under the agreement of extension or deferment; except that a minimum charge of one dollar ($1) for the period of extension or deferral may be made in any case where the extension or deferral agreement charge, when computed at such rate, amounts to less than one dollar ($1). (c) Where the contract includes a finance charge determined on the simple-interest basis, the holder may charge and contract for the payment of an extension or deferral agreement charge by the buyer and collect and receive the same, but the charge for the extension or deferral agreement may not exceed the lesser of twenty-five dollars ($25) or 10 percent of the then unpaid balance of the contract. Such charge shall be in addition to any finance charges which accrue because such extended or deferred payments are received at a time other than as originally scheduled. (d) An extension or deferral agreement may also provide for the payment by the buyer of the additional cost to the holder of the contract of premiums for continuing in force, until the end of such period of extension or deferral, any insurance coverages provided for in the contract, subject to the provisions of Section 1803.5.