CA Gov't Code Section 23202

A boundary line between counties shall not be changed, pursuant to this article, if the change would result in any of the following:

(a)

The line to be moved a distance of more than 5 miles from its original location.

(b)

A reduction by more than 5 percent in the area of any affected county.

(c)

A reduction by more than 5 percent of the population of any affected county, as determined by the State Department of Finance’s most recent estimate of population.