California Code, Revenue and Taxation Code - RTC § 43155

(a) Any person who fails to pay any tax, installment, or prepayment, except amounts of determinations made by the California Department of Tax and Fee Administration under Article 2 (commencing with Section 43201), within the time required shall pay a penalty of 10 percent of the tax, installment, or prepayment, plus interest at the modified adjusted rate per month, or fraction thereof, established pursuant to Section 6591.5, from the date on which the tax or the amount of tax required to be collected became due and payable to the state until the date of payment.

(b) Any person who fails to file a return with the California Department of Tax and Fee Administration in accordance with this part within the time prescribed for the filing of a return, a penalty of 10 percent of the amount of tax, exclusive of previously remitted installments, shall be added thereto on account of the delinquency.

(c) The penalties imposed by this section shall be limited to a maximum of 10 percent of the taxes for which the return, installment, or prepayment is required for any one return, installment, or prepayment.

(d) The amendments made to this section by the act adding this subdivision  1 shall apply to any return or payment that becomes due on or after November 30, 2024. This section does not apply to any fee imposed pursuant to Section 25205.5 or 25205.5.2 of the Health and Safety Code for any return or payment that becomes due on or after November 30, 2024.