Cal. Sts. & High. Code § 5108.5

If an assessment district is created pursuant to Section 5180, the legislative body shall include the assessments imposed by Section 5108.3 with the assessments imposed by Section 5180 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 5108.3.