California Code, Health and Safety Code - HSC § 13851

(a) Within 30 days after the date of filing a petition, the secretary of the district board shall cause the petition to be examined and shall prepare a certificate of sufficiency indicating whether the petition is signed by the requisite number of signers.

(b) The secretary shall cause the names of the signers on the petition to be compared with the voters' register in the office of the county clerk or registrar of voters and ascertain (i) the number of registered voters in the district, and (ii) the number of qualified signers appearing upon the petition.

(c) If the certificate of the secretary shows the petition to be insufficient, the secretary shall immediately give notice by certified mail of the insufficiency to the chief petitioners. That mailed notice shall state in what amount the petition is insufficient. Within 15 days after the date of the notice of insufficiency, the chief petitioners may file with the secretary a supplemental petition bearing additional signatures.

(d) Within 10 days after the date of filing a supplemental petition, the secretary shall examine the supplemental petition and certify in writing the results of his or her examination.

(e) The secretary shall sign and date a certificate of sufficiency. That certificate shall also state the minimum signature requirements for a sufficient petition and show the results of the secretary's examination. The secretary shall mail a copy of the certificate of sufficiency to the chief petitioners.

(f) Once the chief petitioners have filed a sufficient petition, the district board shall take the actions required pursuant to Section 13845, 13846, or 13848.