California Code, Revenue and Taxation Code - RTC § 19131

(a) If any taxpayer fails to make and file a return required by this part on or before the due date of the return or the due date as extended by the Franchise Tax Board, then, unless it is shown that the failure is due to reasonable cause and not due to willful neglect, 5 percent of the tax shall be added to the tax for each month or fraction thereof elapsing between the due date of the return (determined without regard to any extension of time for filing) and the date on which filed, but the total penalty may not exceed 25 percent of the tax. In the case of a commencing corporation, the penalty shall apply to all tax accruable on the due date of the return. The penalty so added to the tax shall be due and payable upon notice and demand from the Franchise Tax Board.

(b) In the case of an individual or fiduciary who fails to file a return of tax required by this part within 60 days of the date prescribed for filing of that return (determined with regard to any extension of time for filing), unless it is shown that the failure is due to reasonable cause and not due to willful neglect, this penalty may not be less than the lesser of one hundred thirty-five dollars ($135) or 100 percent of the amount of tax required to be shown on the return.

(c) For purposes of this section, the amount of tax required to be shown on the return shall be reduced by the amount of any part of the tax which is paid on or before the date prescribed for payment of the tax and by the amount of any credit against the tax which may be claimed upon the return.

(d) If any failure to file any return is fraudulent, subdivision (a) shall be applied by:

(e) This section does not apply to any failure to pay any estimated tax required by Section 19025 or 19136.