Revenue and Taxation Code section 6591
(a)
Any person who fails to pay any tax to the state or any amount of tax required to be collected and paid to the state, except amounts of determinations made by the board under Article 2 (commencing with Section 6481) or Article 3 (commencing with Section 6511) of this chapter, within the time required shall pay a penalty of 10 percent of the tax or amount of the tax, in addition to the tax or amount of tax, 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 in accordance with the due date set forth in Section 6451 or the due date established by the board in accordance with Section 6455, shall pay a penalty of 10 percent of the amount of taxes, 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 taxes for which the return is required, exclusive of any prepayments, for any one return.
Source:
Section 6591, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=RTC§ionNum=6591. (accessed Jun. 2, 2025).