California Code, Revenue and Taxation Code - RTC § 11430

If the additional tax is not paid within the time specified in Section 11429, it is delinquent and a penalty of 10 percent of the amount of the additional tax shall be added thereto, plus interest on the amount of the additional tax at the adjusted annual rate established pursuant to Section 19521 from the date on which the additional tax became due and payable until the time of payment.