California Code, Revenue and Taxation Code - RTC § 19067

(a) Where before the expiration of the time prescribed for the mailing of a notice of a proposed deficiency assessment, the taxpayer consents in writing to an assessment after that time, the assessment may be made at any time prior to the expiration of the period agreed upon. The period agreed upon may be extended by subsequent agreements in writing made before the expiration of the period previously agreed upon.

(b) The Franchise Tax Board shall notify the taxpayer of the taxpayer's right to refuse to extend the expiration of the time prescribed for the mailing of a notice of a proposed deficiency assessment, or to limit that extension to a particular period of time, on each occasion when the taxpayer is requested to provide the taxpayer's consent.

(c) The amendments made by the act adding this subdivision shall apply to any request to extend the expiration of the time prescribed for the mailing of a notice of a proposed deficiency assessment made after December 31, 2000.