Marin Municipal Water District Backflow Letter?

Conor Crawford • February 23, 2026

What Marin Property Owners Need To Know (2026 Guide)

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: