California Code, Government Code - GOV § 65010

(a) Formal rules of evidence or procedure applicable in judicial actions and proceedings shall not apply in any proceeding subject to this title except to the extent that a public agency otherwise provides by charter, ordinance, resolution, or rule of procedure.

(b) No action, inaction, or recommendation by any public agency or its legislative body or any of its administrative agencies or officials on any matter subject to this title shall be held invalid or set aside by any court on the ground of the improper admission or rejection of evidence or by reason of any error, irregularity, informality, neglect, or omission (hereafter, error) as to any matter pertaining to petitions, applications, notices, findings, records, hearings, reports, recommendations, appeals, or any matters of procedure subject to this title, unless the court finds that the error was prejudicial and that the party complaining or appealing suffered substantial injury from that error and that a different result would have been probable if the error had not occurred. There shall be no presumption that error is prejudicial or that injury was done if the error is shown.