Cal. RTC § 19057

(a) Except in the case of a false or fraudulent return and except as otherwise expressly provided in this part, every notice of a proposed deficiency assessment shall be mailed to the taxpayer within four years after the return was filed. No deficiency shall be assessed or collected with respect to the year for which the return was filed unless the notice is mailed within the four-year period or the period otherwise provided. For purposes of this chapter, the term “return” means the return required to be filed by the taxpayer and does not include a return of any person from whom the taxpayer has received an item of income, gain, loss, deduction, or credit.

(b) The running of the period of limitations provided in subdivision (a) on mailing a notice of proposed deficiency assessment shall, in a case under Title 11 of the United States Code, be suspended for any period during which the Franchise Tax Board is prohibited by reason of that case from mailing the notice of proposed deficiency assessment and for 60 days thereafter.

(c) Where, within the 60-day period ending on the day on which the time prescribed in this section for the assessment of any tax imposed under Part 10 (commencing with Section 17001) or Part 11 (commencing with Section 23001) for any taxable year would otherwise expire, the Franchise Tax Board receives a written document, other than an amended return or a report required by Section 18622, signed by the taxpayer showing that the taxpayer owes an additional amount of that tax for that taxable year, the period for the assessment of an additional amount in excess of the amount shown on either an original or amended return shall not expire before the day 60 days after the day on which the Franchise Tax Board receives that document.

(d) If a taxpayer determines in good faith that it is an exempt organization and files a return as an exempt organization under Section 23772, and if the taxpayer is thereafter held to be a taxable organization for the taxable year for which the return is filed, that return shall be deemed the return of the organization for the purposes of this section.