Cal. Gov. Code § 23600
The county seats of the respective counties of the state, as fixed by law and designated in this article, are declared to be the county seats of the respective counties. In any case where a county seat is an incorporated city, it includes all territory heretofore or hereafter annexed to the incorporated city.
The board of supervisors shall designate by resolution an alternative temporary county seat, which may be outside the boundaries of the county, for use in the event of war or enemy-caused disaster, or the imminence thereof, but real property outside the boundaries of the county shall not be purchased by a county for use as a temporary county seat. A copy of the resolution shall be filed with the appropriate county officials in that county and the alternative county. A different temporary county seat may be so designated at any time as circumstances indicate the desirability of such a change.
The board, and any county officer or agency as directed by the board, shall provide such facilities of any kind at the temporary county seat as appear desirable for the functioning of the government of the county at the temporary county seat in the event that it becomes necessary, pursuant to this section.
Ca. Gov. Code § 23600