By Conor Crawford
•
February 23, 2026
If you recently received a “Notice to Install Backflow Prevention Assembly” from Marin Municipal Water District (MMWD) — you are not alone. MMWD is actively issuing letters to residential and commercial property owners throughout Marin County requiring the installation of an approved reduced pressure (RP) backflow preventer at the water meter or hazard connection point. If you own property in: San Rafael, Mill Valley, Sausalito, Tiburon, Belvedere, Corte Madera, Larkspur, Kentfield, Ross, San Anselmo, Fairfax, Novato, Greenbrae, Strawberry, Woodacre, Lagunitas, Forest Knolls, Point Reyes Station, Inverness, Stinson Beach, Bolinas, Olema, Nicasio, Marshall, Tomales, Dillon Beach, or anywhere in Marin — this applies to you. Let’s break down exactly what this letter means, why you’re receiving it, and how to stay compliant. Why Is MMWD Requiring Backflow Prevention? Under California Code of Regulations (Titles 17 & 22) and the California Plumbing Code, water districts are required to protect the public drinking water system from contamination. Backflow occurs when water flows in the wrong direction due to: Back-siphonage (pressure drops in the main) Backpressure from equipment on private property Irrigation systems Fire sprinkler systems Boilers and hydronic heating systems Commercial plumbing equipment Cross-connections To prevent contamination, MMWD requires installation of an approved Reduced Pressure (RP or RPZ) backflow prevention assembly. This is not optional once you receive the letter. What The MMWD Letter Typically Requires Based on current enforcement letters being sent across Marin County, property owners must: Install an MMWD-approved reduced pressure backflow preventer Install it at the water meter (or designated hazard location) Ensure proper clearance for testing and maintenance Have it tested immediately after installation Submit results via MMWD’s reporting system Test the device annually thereafter Complete installation before the listed deadline Failure to comply can result in water service issues or enforcement action. Important MMWD Installation Requirements (That Many Contractors Miss) The MMWD standard installation diagram contains strict requirements. A non-compliant installation will fail inspection or testing. Key requirements typically include: Must Be Installed On Customer Property Backflow assemblies must be installed downstream of the water meter on the customer’s side. No Connections Between Meter and Backflow MMWD generally prohibits: Tees Unions Hose bibs Bypass piping Strainers PRVs before the assembly (unless specifically approved) The line must go directly from meter to backflow assembly. ✔ Proper Clearance Assemblies must have: Minimum clearance around the device (commonly 12") Adequate space for certified tester access Installation height typically between 12"–30" above finished grade ✔ Approved Assembly Only The device must be on MMWD’s approved list. ✔ Annual Testing Required Once installed, the assembly becomes part of the District’s compliance program. Why So Many Marin Property Owners Are Getting Letters Over the last few years, water districts across California have increased cross-connection enforcement. In Marin specifically, letters are often triggered by: Irrigation systems tied into domestic water Fire sprinkler systems Hydronic heating systems with fill connections Commercial tenant improvements Property sales and surveys Cross-connection inspections Many older homes in San Rafael, Mill Valley, Kentfield, and Ross were never originally required to install an RP — enforcement is catching up. How Much Does Backflow Installation Cost in Marin County? Costs vary depending on: Size of water service Meter box configuration Need for concrete or asphalt cutting Bollard protection (if in driveway or parking area) Relocating PRV after backflow Accessibility constraints In Marin County, residential installations commonly range from moderate to significant investment depending on conditions, while commercial properties vary widely. The key is installing it correctly the first time. Backflow Installation in Marin County – Town By Town Local Plumbing installs MMWD-compliant RP backflow assemblies throughout: Central Marin San Rafael Corte Madera Larkspur Kentfield Greenbrae Ross San Anselmo Fairfax Southern Marin Mill Valley Sausalito Tiburon Belvedere Strawberry Marin City Northern Marin Novato Nicasio West Marin Point Reyes Station Inverness Olema Stinson Beach Bolinas Woodacre Lagunitas Forest Knolls Marshall Tomales Dillon Beach Each town presents unique site challenges — from hillside properties in Mill Valley to tight meter boxes in Tiburon to coastal corrosion in Stinson Beach. What Happens After Installation? Immediately after installation: A certified tester must test the assembly. Results must be submitted to MMWD. You’ll enter the annual testing cycle. We recommend setting calendar reminders so you don’t receive violation notices. Frequently Asked Questions Do I really need this if my neighbor doesn’t have one? If you received a letter, yes. Enforcement is based on survey findings and hazard classification. Can I install a cheaper double check valve? Not if MMWD requires a Reduced Pressure (RP) assembly. Will this reduce my water pressure? It can slightly affect pressure depending on existing conditions. A proper system evaluation prevents performance issues. Is this required for residential homes? Yes — many single family homes are now receiving letters. Why Choose Local Plumbing For Backflow Installation? Local Plumbing specializes in: Code-compliant installations Clean meter re-piping Proper clearance layouts Coordination with certified testers Pressure regulator integration Marin County permitting familiarity Commercial and residential experience We install systems that are: Clean Serviceable Built to pass inspection Designed for long-term reliability Received an MMWD Backflow Letter? Don’t Wait. Deadlines matter. Testing schedules fill quickly. If you’ve received a notice to install a reduced pressure backflow preventer at your Marin County property, contact Local Plumbing today. We’ll: Review your MMWD letter Confirm exact requirements Provide a clear installation plan Ensure compliance before your due date Local Plumbing Your Local Marin Plumber Backflow Prevention • Boiler Systems • Water Services • Code Compliance Serving All of Marin County