California Code, Revenue and Taxation Code - RTC § 11316

If the board makes an assessment pursuant to Section 11311, 11314, or 11315 due to the negligence of the taxpayer, a penalty of 10 percent of the value of the estimated or escape assessment shall be added to the assessment. If the estimated or escape assessment is due to a fraudulent or willful attempt to evade the tax, a penalty of 25 percent of the value of the estimated or escape assessment shall be added to the assessment. A willful failure to file a report as required by Article 2 (commencing with Section 11271) of this chapter shall be deemed to be a willful attempt to evade the tax.

If the assessee establishes to the satisfaction of the board that the failure to file an accurate property statement was due to 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.