Does my ADU need a water pressure regulator (PRV)?

Your ADU needs a pressure regulator if static water pressure in its supply piping is over 80 psi. Section 608.2 of the 2025 California Plumbing Code requires an approved regulator, preceded by a strainer, to bring pressure down to 80 psi or less. If the main house already has a working regulator and the ADU line branches off after it, the ADU may already be covered.

How to find out

• Ask your water purveyor for the pressure range at your meter. The code's sizing method starts with available pressure at the meter, and where pressure varies during the day you design for the minimum (CPC 610.7 and 610.8).
• Put a gauge on a hose bibb near the meter to read static pressure. Pressure can rise at night when demand drops.
• Look for an existing regulator where the service enters the house, and note whether the ADU's tap is upstream or downstream of it.

What the code requires when a regulator goes in

• The regulator must comply with ASSE 1003 or AWWA C530 for potable water.
• A strainer goes ahead of it, except on regulators 1-1/2 inches and larger. The strainer must be cleanable without removing the regulator or disconnecting the piping.
• The regulator and strainer must be accessible, above ground or in a drained vault, and protected from freezing.
• An approved expansion tank goes in the cold water piping downstream of each regulator, so thermal expansion from the water heater can't push pressure above 80 psi. A system designed by a licensed plumbing contractor or registered design professional may use pressure relief valves set at 100 psi or less instead.
• When sizing pipe with Table 610.4, use 80 percent of the reduced pressure.

Low pressure is a different problem

A detached ADU at the back of a lot can end up with too little pressure rather than too much. Long developed length, elevation gain, and losses through meters, backflow devices, filters, softeners, and tankless water heaters all reduce what reaches the fixtures (CPC 610.2 and 610.7). The fix is correct pipe sizing, and occasionally a booster pump, not a regulator. If you add a booster, it needs a low-pressure cutoff (CPC 609.9).

Why it matters for fixtures

CALGreen flow limits are rated at set test pressures, for example 60 psi for faucets and 80 psi for showerheads. Pressure held within the code range keeps valves, supply connectors, and the water heater out of trouble and keeps fixtures performing as rated.

What it means for your plumbing plan

Your plans should state the design pressure, show any new regulator and expansion tank (or note the existing ones), and size the ADU supply line from fixture units, developed length, and elevation. See fixture unit counts, separate water meters, and our plumbing plans.

Sources: gocodebook.com, gocodebook.com, gocodebook.com.