Cal. Code Regs. tit. 18 § 15801.1
In a case not involving a false or fraudulent return or failure to file a return, and where the return is filed within the time specified by law, the Controller may issue the deficiency determination at any time after the tax is due (i.e., the 15th day of the second month following the close of the calendar quarter in which the gift is made), but no later than four (4) years after the last day prescribed by law for filing the return; provided, however, the Controller and the donor or other person liable for the tax may agree in writing that the deficiency determination may be made after the four-year period, in which case the determination shall be issued within the period agreed upon.
Where the Controller and the donor or other person liable for the tax agree in writing that the determination is to be made at some time after the expiration of the four-year period, the parties may from time to time agree to extend the time within which the determination may be issued.
NOTE: Reference: Section 15801, Revenue and Taxation Code.
Cal. Code Regs. Tit. 18, § 15801.1