Cal. Gov. Code § 23285
Whenever county boundaries are changed pursuant to this article, the board of supervisors of both affected counties shall cause to be filed before the following December 1, with the State Board of Equalization and with the assessors of both affected counties, a statement setting forth the legal description of the boundaries, as changed, together with a map or plat indicating those boundaries. The change of the boundaries shall not be effective for purposes of assessment or taxation unless the statement, together with the map or plat required by this section, is filed with the county assessors and with the State Board of Equalization on or before December 1 of the year immediately preceding the year in which the assessments or taxes are to be levied.
Ca. Gov. Code § 23285