Cal. Pen. Code § 13902
Each county placed within a single county planning district may constitute a planning district upon execution of a joint powers agreement or arrangement acceptable to the county and to at least that one-half of the cities in the district which contain at least one-half of the population of the district. Counties placed within a multicounty planning district may constitute a planning district upon execution of a joint powers agreement or other arrangement acceptable to the participating counties and to at least that one-half of the cities in such district which contain at least one-half of the population of such district. If no combination of one-half of the cities of a district contains at least one-half of the population of the district, then agreement of any half of the cities in such district is sufficient to enable execution of joint powers agreements or other acceptable arrangements for constituting planning districts.
Ca. Pen. Code § 13902