Do I need a sewage ejector or grinder pump for my ADU, and what does code require?

You need a pumped sewer connection only when your ADU's drains sit too low to flow by gravity to the sewer. The 2025 California Plumbing Code (CPC) says fixtures below the crown of the main sewer must drain into a watertight sump, and an ejector, grinder pump, or similar device then lifts the waste to the gravity line. An ejector moves solids whole; a grinder pump chops them first so it can push through smaller pipe.

When gravity won't work

Gravity drainage needs continuous downhill slope from every fixture to the tie-in. An ADU usually loses that slope when:

• A detached ADU sits downhill from the house or the street sewer.
• A long run to the tie-in uses up the available fall before the pipe reaches the lateral.
• A conversion puts a bathroom below grade, such as in a basement.
• The existing lateral is shallow, leaving no room to bring the new building sewer in below it.

Before you commit to a pump, confirm the depth of the existing lateral where you plan to connect. A camera inspection and a few measured elevations often settle the question.

Ejector vs. grinder: what the code requires

Sewage ejector (CPC 710.3). When a pump in a single-dwelling unit receives toilet waste, it must pass a 1-1/2 inch solid ball, discharge at least 20 gallons per minute, and use discharge piping of at least 2 inches with a backwater (check) valve and a gate valve. The shutoff goes on the discharge side of the check valve, and the discharge line enters a horizontal gravity drain from the top through a wye (710.4).

Grinder pump (CPC 710.12). Grinder pumps are expressly permitted. Discharge piping is sized per the manufacturer but not smaller than 1-1/4 inches, with a check valve and a full-way shutoff valve on the discharge line. The smaller pipe can help on long or uphill runs.

Macerating toilet systems (CPC 710.13). Where your building official approves, a listed macerating system can be used instead of a sewage pump system. Ask early, because acceptance for a full ADU varies.

Sump, vent, and alarm details

• The sump must be watertight with a gasketed, bolted cover, and the lowest inlet must sit at least 2 inches above the high-water or starting level (710.8 through 710.10).
• The sump vent must keep the basin at atmospheric pressure and can't be smaller than 1-1/2 inches (710.10).
• Many building departments and pump manufacturers call for an audible and visual high-water alarm that signals a pump failure, so plan for one and confirm what your jurisdiction requires. Plan for one.
• Pump flow counts toward downstream pipe sizing: the code assigns two fixture units per gallon per minute of pump capacity (710.5).
• The pump needs power per the manufacturer's instructions, so it belongs on the electrical plan and load calculation.

What it means for your plumbing plan

Your plans should show the sump location, pump type and model, discharge pipe size and route, check and shutoff valves, vent, alarm, and where the pressure line joins the gravity sewer. Decide before the site plan is fixed: shifting the ADU or raising its floor can sometimes restore gravity flow and avoid a pump entirely. Our plumbing plans document either path, and you can compare connection options in how ADU sewer connection works and the ADU plumbing guide.

Sources: gocodebook.com.