Cal. ELEC § 4004

(a) “Small city” means a city with a population of 100,000 or less, as determined by the annual city total population rankings by the Demographic Research Unit of the Department of Finance. (b) “Eligible entity” means a school district or a special district. (c) Notwithstanding Sections 1500 and 4000, an election in a small city or an eligible entity may be conducted wholly as an all-mail ballot election, subject to the following conditions: (1) The legislative body of the small city or the governing body of the eligible entity, by resolution, authorizes the use of mailed ballots for the election. (2) The election is a special election to fill a vacancy in the legislative body or governing body. (3) The election is not held on the same date as a statewide primary or general election. (4) The election is not consolidated with any other election. (5) The return of voted mail ballots is subject to Section 3017.