California Code, Revenue and Taxation Code - RTC § 19108

(a) Where an overpayment is made by any taxpayer for any year, and a deficiency is owing from the same taxpayer for any other year, the overpayment, if the period within which credit for the overpayment may be allowed has not expired, shall be credited on the deficiency, if the period within which assessment of the deficiency may be proposed has not expired, and the balance, if any, shall be credited or refunded to the taxpayer. No interest shall be assessed on the portion of the deficiency as is extinguished by the credit for the period of time subsequent to the date the overpayment was made.

(b) For the purposes of this section the returns of a decedent and his or her estate shall be considered returns of the same taxpayer and the returns for the decedent and his or her estate filed for the year of death shall be considered returns for different taxable years.

(c) This section is not intended, nor shall it be construed, as a limitation on the Franchise Tax Board's right to offset or recoup barred assessments against overpayments.