Cal. Rev. & Tax. Code § 45203

If, before the expiration of the time prescribed in Section 45202 for serving a notice of deficiency determination, the fee payer has consented in writing to the service of the notice after that time, the notice may be served 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.