California Code, Probate Code - PROB § 1000

(a) Except to the extent that this code provides applicable rules, the rules of practice applicable to civil actions, including discovery proceedings and proceedings under Title 3a (commencing with Section 391) of Part 2 of the Code of Civil Procedure, apply to, and constitute the rules of practice in, proceedings under this code. All issues of fact joined in probate proceedings shall be tried in conformity with the rules of practice in civil actions.

(b) For purposes of determining when a petitioner in a proceeding under this code may commence discovery as to nonparties, the time periods set forth in the Civil Discovery Act (Title 4 (commencing with Section 2016.010) of Part 4 of the Code of Civil Procedure) shall apply, except that the time periods shall commence to run upon service of the petition and notice of hearing upon all parties entitled to notice. Nothing in this subdivision shall either alter when a respondent in such a proceeding may commence discovery or increase the extent to which nonparties may be subject to discovery.