California Code, Revenue and Taxation Code - RTC § 733

(a) If a timely petition for reassessment is not filed with the board, an assessment of unitary or nonunitary property of the assessee shall become final at the expiration of the period specified for filing a petition in the notice given in accordance with Section 731 or Section 732.

(b) The board may extend the period for filing a petition for reassessment once for a period not to exceed 15 days, provided a written request for the extension is filed with the board prior to the expiration of the period for which the extension may be granted.