Plumbing guide · San Jose, CA
What ADU plumbing takes in San Jose, CA
An accessory dwelling unit is a small house, and it needs everything a house needs: a sewer connection, a water supply sized for two households, hot water, and often gas or a plan to go all-electric. Most of it has to be decided before the foundation goes in.
By Earl Newcomb, Plumbing Guides Editor · Updated September 2026
ADU plumbing in San Jose covers the sewer connection to the main lateral or a new one, a water supply sized for both units, a separate water heater, and gas or all-electric appliances, all permitted and inspected. The decisions that cost the most to change, like where the sewer line runs and how big the water supply is, have to be made before the slab is poured.
Why ADU plumbing is planned early
The City of San José reports roughly 1,500 accessory dwelling units permitted since 2022, and ADU permits there have doubled in five years. Almost all of them share the same lesson: the plumbing has to be designed before the foundation. Drain lines run under the slab at a fixed slope, so the ADU's floor height and the distance to the sewer decide whether the plan works at all.
The sewer connection
Most ADUs tie into the main house's existing sewer lateral, which means that lateral has to be in good enough shape to carry a second household. A camera inspection before design shows whether it is sound, cracked or full of roots. In San Jose the homeowner owns the upper lateral from the house to the property line, and the city requires a permit and inspection for lateral work, so any repair is best done as part of the ADU project.
If the ADU sits low on the lot or far from the main line, the drain may need a different route, a cleanout at the junction, or in rare cases a pump.
Ready when you are
Designing an ADU?
Call before the foundation is poured, and we will check the lateral and the water supply.
Water supply and pressure
The existing water service and meter have to supply both units. Long runs to a backyard unit lose pressure, and an undersized line makes the ADU's shower weak whenever the main house runs water. Upsizing the line in the same trench as the sewer is usually cheaper than fixing it later; where the street pressure itself is low, a booster pump is the other option.
Hot water and gas
An ADU needs its own water heater. With the Bay Area's zero-NOx rule for new water heaters taking effect in 2027, a heat pump or other electric water heater is often the simplest choice, and it avoids running a gas line and flue to the unit. If the ADU will have gas cooking or heat, the gas line is sized and pressure-tested along with the rest.
Permits, inspections and timing
ADU plumbing is inspected at rough-in, before walls close, and again at final. The plumbing timeline follows the build: sewer and water trenching before the slab, rough-in after framing, fixtures after finishes. Our ADU plumbing page covers how we run each stage.
Cost figures on this page are planning ranges from published guides, not quotes: heritageplumbingsanjose.com (2026-01-30); heritageplumbingsanjose.com (2026-01-30). Every job is priced after we see it.
Related services
Where this guide leads
More guides: SB 407 and water-conserving fixtures, Sewer lateral replacement cost, Tankless vs tank water heaters.
FAQ
Questions readers ask
Usually, if the lateral is in good shape; a camera inspection tells you.
Not always; the existing service has to be sized for both units.
Rarely well; most ADUs get their own, often a heat pump unit.
Before the foundation design is final.
Yes, at rough-in and at final.
Planning an ADU in San Jose?
Call any hour to talk through the sewer, water and hot water plan.
Updated September 2026