California Code, Government Code - GOV § 12410.6

(a) An audit for any local agency, including those submitted to the Controller pursuant to subdivision (a) of Section 12410.5, shall be made by a certified public accountant or public accountant, licensed by, and in good standing with, the California Board of Accountancy.

(b) Commencing with the 2013-14 fiscal year, a local agency shall not employ a public accounting firm to provide audit services to a local agency if the lead audit partner or coordinating audit partner having primary responsibility for the audit, or the audit partner responsible for reviewing the audit, has performed audit services for that local agency for six consecutive fiscal years.  For purposes of calculating the six consecutive fiscal years, the local agency shall not take into account any time that a public accounting firm was employed by that local agency prior to the 2013-14 fiscal year.  The Controller may waive this requirement if the Controller finds that another eligible public accounting firm is not available to perform the audit.