A county may not form an area within the territorial jurisdiction of a city without the consent of the city council of that city. A city may not form an area within the unincorporated territory of a county, without the consent of the board of supervisors of that county. A city may not form an area within the territorial jurisdiction of another city without the consent of the city council of the other city.
Source:
Section 36521.5, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=SHC§ionNum=36521.5..