Cal. Sts. & High. Code § 10125

If an assessment district is created pursuant to Section 10200, the legislative body shall include the assessments imposed by Section 10123 with the assessments imposed by Section 10200 within an assessment district created to pay the costs of a water supply system, or a sanitary sewer and facilities project, where the costs of proposed improvements for such a project have been approved pursuant to Section 10123.