Cal. Rev. & Tax. Code § 2056

Whenever it becomes necessary for any purpose otherwise required by law to determine the assessed value of public property or other property exempt from taxation, as such assessed value is shown on the last equalized roll, and no value is shown on the roll for such property, and there is no other applicable provision for determining such assessed value, the assessor shall, upon request, provide in writing an estimate of the value according to his current assessment practices, and such estimate shall be deemed to be the assessed value for such purpose.