Cal. Health & Saf. Code § 4894.1
When the board of supervisors has so declared in its resolution of intention to order the formation of a sewer maintenance district or any annexation thereto, the board of supervisors may, in its resolution ordering the formation of a sewer maintenance district or any annexation thereto, order the district divided into tax assessment zones. Tax assessment zones may be formed at a time other than at the formation of the district or any annexation thereto if pursuant to the same procedure of resolution, notice, and hearing as are applicable under this chapter to the annexation of territory.
No district shall be divided into zones unless the board of supervisors finds that a tax assessment zone requires special services or special facilities in addition to those provided generally by the sewer maintenance district or the replacement of obsolete facilities or equipment, and that the tax levy is commensurate with the special benefits to be provided in the zone. The boundaries of tax assessment zones may be changed or a zone may be dissolved in the same manner that a zone may be formed in an existing district.
Ca. Health and Saf. Code § 4894.1