Cal. RTC § 532.1

(a) If, before the expiration of the period specified in Section 532 for making an escape assessment, the taxpayer and the assessor have agreed in writing to extend the time for making an assessment, correction, or claim for refund, the assessment may be made at any time prior to the expiration of the period agreed upon. The period may be extended by subsequent agreements in writing made before the expiration of the period previously agreed upon.

(b) If the assessor mails or otherwise delivers a “Notice of Proposed Escape Assessment” under Section 531.8 as to any assessment year for which the period for making an escape assessment, including any extension pursuant to subdivision (a), will expire in less than 90 days after the date of mailing or delivery of that notice, then that period, and any limitations period on the filing of a refund claim with respect to the same assessment year, shall be extended to the 90th day after the date of mailing or delivery. Subsequent mailings or deliveries of a “Notice of Proposed Escape Assessment” for the same assessment year shall not establish any further extension.