San Francisco Building Inspection Commission Codes § 606.11 Add the following section

606.11 Backflow Assembly. All buildings four stories or more in height or water supply greater than 40 feet above the water meter shall have an approved backflow assembly installed as near as possible to the water meter and before the first fitting or branch line. The backflow assembly installed shall be in accordance to the degree of hazard within the building, but in no case less than an approved double check assembly. See Table 603.2.