California Code, Revenue and Taxation Code - RTC § 11273

If any person required to file a report fails to file it on or before April 30 or at the time as extended by the board, a penalty of 10 percent of the assessed value shall be added to the assessment.

If the assessee establishes to the satisfaction of the board that the failure to file the property statement timely was due to a reasonable cause and occurred notwithstanding the exercise of ordinary care and the absence of willful neglect, the board shall order the penalty abated, provided the assessee has filed with the board written application for abatement of the penalty within the time prescribed by law for filing a petition for reassessment.