Cal. Rev. & Tax. Code § 55064

If before the expiration of the time prescribed in Section 55062 for the mailing of a notice of deficiency determination the feepayer has consented in writing to the mailing of the notice after that time, the notice may be mailed at any time prior to the expiration of the period agreed upon. The period so agreed upon may be extended by subsequent agreements in writing made before the expiration of the period previously agreed upon.