Cal. Health & Saf. Code § 6518
It may lay its sewers and drains in any public street or road in the county, and for this purpose enter upon it and make all necessary and proper excavations, restoring it to proper condition. The work of restoring and repairing any such public street or road in the county shall be done under the supervision and control of the county engineer or road commissioner at the cost of the district, and in accordance with the standards established by ordinance of the board of supervisors for restoring and repairing county roads. If the street or road is in a city the consent of the proper city authorities shall first be obtained. If the street or road is in the unincorporated area of the county, the consent of the proper county authorities shall first be obtained.
Ca. Health and Saf. Code § 6518