California Code, Public Resources Code - PRC § 43203

(a) The board of supervisors of the county may designate the enforcement agency to carry out this chapter in the county.  The designation is subject to the approval by a majority of the cities within the county which contain a majority of the population of the incorporated areas of the county, except in those counties which have only two cities, in which case the designation shall be subject to approval by the city which contains the majority of the population of the incorporated area of the county.

(b) The county and the cities within the county may enter into a joint exercise of powers agreement pursuant to Chapter 5 (commencing with Section 6500) of Division 7 of Title 1 of the Government Code for the purpose of establishing an enforcement agency to carry out this chapter in the jurisdiction of the joint powers agency.

(c) A city council may, at any time, designate an enforcement agency to carry out this chapter in the city.

(d) The board of supervisors of the county may designate an enforcement agency to carry out this chapter in the unincorporated area of the county.