How Does an ADU Connect to Sewer?

Sewer is the utility where physics writes the invoice. Water and power can be pushed anywhere; waste wants to flow downhill, and your lot either cooperates or it doesn't.

The standard connection. Your house already has a sewer lateral — the pipe carrying waste to the city main. Most ADUs tie into that existing lateral at a convenient point, sharing it. Requirements: the lateral must be in decent condition, sized adequately (usually fine), and — the big one — reachable by gravity from the ADU.

The gravity rule. Drain pipes need a continuous downhill slope, typically about ¼ inch per foot. Your plumber measures the depth of the lateral at the connection point and works backward: can the ADU's drains, starting under its slab, maintain that slope over the distance? Flat lots with shallow laterals and distant ADUs are where this gets tight.

When gravity fails: the ejector pump. If the ADU's plumbing sits too low (or too far on too flat a run), waste gets pumped uphill via a sewage ejector pump — a sealed basin and grinder pump. It works fine, but it adds $3K–$8K up front, needs power, and becomes a mechanical component that can fail and must be maintained for the life of the unit. Gravity, once achieved, is free forever. Worth designing for.

Placement is the lever. Moving the ADU 15 feet, flipping the bathroom to the near side, or raising the finish floor a few inches can be the difference between gravity and a pump. This is exactly the kind of decision that must happen at design time — one more reason utilities come before floor plans.

On septic instead of sewer? ADUs on septic are done all the time, but the system's capacity must handle the added bedrooms — which may mean an expanded or new system, a percolation test, and county health review. Start that conversation in feasibility; septic surprises are among the priciest.