Does my ADU need a backwater valve on its sewer line?

Your ADU needs a backwater valve if any of its fixtures are installed on a floor lower than the cover of the next upstream manhole on the sewer that serves it. That is the trigger in Section 710.1 of the 2025 California Plumbing Code. When a sewer main backs up, sewage can rise to about the level of that manhole cover, so anything lower can flood.

How to tell if the rule applies

1. Find the manhole on the sewer main immediately upstream of where your lateral connects. Upstream means the direction the flow comes from.
2. Get the elevation of that manhole cover. Your city or sewer agency may have it, or a surveyor can shoot it.
3. Compare it with the finished floor of the ADU's lowest fixtures, usually the shower, tub, or toilet.
4. If that floor is lower than the manhole cover, the drains serving those fixtures need a backwater valve.

Downhill lots, ADUs that sit below the street, and lower-level conversions are the usual cases. A detached ADU at the back of a lot that slopes away from the street is a classic example.

Installation rules

• Protect only the low fixtures. Fixtures on floors above the manhole-cover elevation must not drain through the backwater valve (710.1). That keeps upper fixtures draining when the valve is closed.
• Keep it accessible. Backwater valves must be accessible for inspection and repair, or set in a pit with a removable cover (710.6). A valve buried under a slab with no access defeats the purpose.
• Use a listed valve. Valves must comply with ASME A112.14.1 or IAPMO IGC 305 (710.6).
• Label the cleanouts. Cleanouts on drains that pass through a backwater valve must carry a permanent label warning that a backwater valve is downstream, so a drain cleaner doesn't damage it.

Below the sewer itself? That's a pump

A backwater valve protects fixtures that can still drain by gravity but sit below the upstream manhole cover. If fixtures sit below the crown of the sewer main, gravity can't work at all, and the code requires a sump with an ejector or grinder pump, which has its own check valve on the discharge line (710.2 through 710.4).

Local rules can add more

Some cities and sewer agencies layer their own process on top of the state code. The City of Santa Barbara, for example, ties its Section 710.1 backwater valve requirement to permitted plumbing additions and alterations and asks a licensed plumbing contractor to certify whether a valve is required. Ask your building department and sewer agency whether they have a local form, standard detail, or manhole elevation data.

What it means for your plumbing plan

Your plans should state the upstream manhole cover elevation (or how it was determined), the ADU's finished floor elevation, and, where needed, the backwater valve's location, size, access cover, and the fixtures it protects. Show it on the DWV layout and the site plan. Our plumbing plans include this check, and the ADU plumbing guide covers the rest of the sewer connection.

Sources: gocodebook.com, santabarbaraca.gov.