California Code, Revenue and Taxation Code - RTC § 46356

All amounts determined to be due by the board under Article 2 (commencing with Section 46201) or Article 3 (commencing with Section 46251) are due and payable at the time they become final, and, if not paid when due and payable, a penalty of 10 percent of the amount determined to be due shall be added to the amount due and payable.