California Code, Revenue and Taxation Code - RTC § 7655

(a) Any person who fails to pay the amount of tax shown to be due by his or her return on or before the last day of the month following the monthly period to which it relates shall pay a penalty of 10 percent of the tax, together with interest on that tax at the modified adjusted rate per month, or fraction thereof, established pursuant to Section 6591.5, from the date on which the tax became due and payable to the state until the date of payment.

(b) Any supplier who fails to file a return in accordance with the due date set forth in Section 7651shall pay a penalty of 10 percent of the amount of the tax, exclusive of prepayments, with respect to the period for which the return is required.

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