Cal. Rules of Court, rule 8.974

(a) Preliminary opposition

(1) The respondent and real party in interest are not required to file any opposition to the petition unless asked to do so by the appellate division judge.

(2) Within 10 days after the petition is filed, the respondent or any real party in interest may serve and file a preliminary opposition.

(3) A preliminary opposition should contain any legal arguments the party wants to make as to why the appellate division judge should not issue a writ and a statement of any material facts not included in the petition.

(4) Without requesting opposition, the appellate division judge may grant or deny a request for temporary stay, deny the petition, issue an alternative writ or order to show cause, or notify the parties that the judge is considering issuing a peremptory writ in the first instance.

(b) Return or opposition; reply

(1) If the appellate division judge issues an alternative writ or order to show cause, the respondent or any real party in interest, individually or jointly, may serve and file a return (which is a response to the petition) by demurrer, verified answer, or both. If the appellate division judge notifies the parties that he or she is considering issuing a peremptory writ in the first instance, the respondent or any real party in interest may serve and file an opposition.

(2) Unless the appellate division judge orders otherwise, the return or opposition must be served and filed within 30 days after the appellate division judge issues the alternative writ or order to show cause or notifies the parties that it is considering issuing a peremptory writ in the first instance.

(3) Unless the appellate division judge orders otherwise, the petitioner may serve and file a reply within 15 days after the return or opposition is filed.

(4) If the return is by demurrer alone and the demurrer is not sustained, the appellate division judge may issue the peremptory writ without granting leave to answer.

(c) Form of preliminary opposition, return, or opposition

Any preliminary opposition, return, or opposition must comply with rule 8.931(c). If it is filed by an attorney, it must also comply with rule 8.932(b)(3)-(7).