CA Rev & Tax Code Section 50113.2

If, before the expiration of the time prescribed in Section 50113.1 for the mailing of a notice of deficiency determination, the fee payer 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 agreed upon may be extended by subsequent agreements in writing made before the expiration of the period previously agreed upon.