California Code, Revenue and Taxation Code - RTC § 1609.4

On the hearing of the application, the county board may subpoena witnesses and books, records, maps, and documents and take evidence in relation to the inquiry. The assessor may introduce new evidence of full cash value of a parcel of property at the hearing and may also introduce information obtained pursuant to Section 441. If the assessor proposes to introduce evidence to support a higher assessed value than he placed on the roll, he shall, at least 10 days prior to the hearing, inform the applicant of the higher assessed value and the evidence proposed to be introduced and he may thereafter introduce such evidence at the hearing.

No subpoena to take depositions shall be issued nor shall depositions be considered for any purpose by the county board or the assessment appeals board.