Cal. Gov. Code § 51520

Notwithstanding any provisions of this article relating to compensation paid to the county for assessing and collecting taxes for a city the board of supervisors may, upon a finding of public interest, perform such assessment and collection service for a city without compensation from such city; provided that, in a county having a population of 4,000,000 or more, the county shall perform such assessment and collection service for a city without compensation from such city; provided, further, that, in any county in which all cities agree to use of the county assessment roll and agree to permit the county to perform such assessment and collection functions, the county shall perform such assessment and collection services for the cities without compensation from such cities.