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ADU plumbing in San Jose, from the sewer tie-in to the fixtures
An accessory dwelling unit needs everything a house needs on a smaller footprint: a drain that reaches the sewer, water and gas sized for two households, hot water, and fixtures that pass inspection. The plumbing is planned before the foundation, because the sewer route decides where the bathroom can go.
ADU plumbing in San Jose covers the sewer connection from the new unit to the existing lateral or a new one, water and gas lines sized for both homes, a water heater for the unit, and the kitchen and bathroom rough-in. It is permitted and inspected with the ADU, and the sewer route is planned before the slab is poured.
The job
Why the plumbing gets planned first
In a detached ADU, the bathroom and kitchen drains have to reach the sewer with enough fall, and the route from the backyard to the lateral decides where those rooms can sit. Water and gas lines have to carry both households at once, which often means the existing lines are too small. Getting those answers early keeps the design from changing after the foundation is formed.
Adding a bathroom inside the main house is covered under bathroom remodel plumbing. Before any tie-in, the existing lateral is checked with a sewer camera inspection.
Cost factors
What changes the price of ADU plumbing
- Distance to the sewer A long backyard run costs more than a short one.
- Existing lateral An old line may need repair before the tie-in.
- Line upsizing Water or gas upgrades add cost.
- Floor type Cutting an existing slab adds work.
- Fixtures The count and type of fixtures drive rough-in.
Every job is priced after we see it; any published range is for planning only.
Your options
Detached ADU or garage conversion?
Both are common in San Jose; the plumbing differs mostly in distance and floor.
| Detached ADU | Garage conversion | |
|---|---|---|
| Sewer route | New line across the yard to the lateral | Often shorter, near the house drain |
| Floor | Plumbed before a new slab is poured | Existing slab cut for new drains |
| Water and gas | Lines trenched across the yard | Branch from lines already at the house |
| Water heater | Its own unit, often tankless or heat pump | Own unit or relocated, depending on layout |
| Typical challenge | Enough fall over a long run | Cutting and patching the garage slab |
Detached ADU sewer tie-in
A detached ADU drains into the property's existing lateral or, where the lot allows, into a new connection of its own. The line has to keep a steady fall from the unit to the tie-in point, so a long, flat backyard run is the first thing we check. We camera the existing lateral before tying in, because an old clay line that barely serves one house will not serve two. Where the unit sits lower than the sewer, a pump system may be needed, and we tell you that before the design is final.
Garage conversion plumbing
Converting a garage puts a bathroom, and often a kitchenette, on a slab that was never meant to have drains in it. The slab is saw-cut along the drain routes and trenched to the right depth, and the new lines tie into the house drain or the lateral. Vents run up through the new walls to the roof. Water and gas branch from nearby lines, and the washer and water heater that often live in a garage may need to move. Everything is inspected before the concrete is patched.
Water and gas line sizing for a second unit
An ADU adds a second kitchen, a second bathroom and usually a second water heater to lines that were sized for one house. We add up the fixture demand for both homes and check whether the existing water service and gas line can carry it. Often the water service or the gas branch has to be upsized, and with a tankless heater, gas sizing is critical. Where the owner wants separate utilities for the unit, we handle the plumbing side of the utility's requirements for a separate meter.
ADU water heater and fixtures
Every ADU needs its own hot water, placed so the unit's fixtures get it quickly. Small units suit tankless or heat pump water heaters, which save space and fit the all-electric designs many ADU owners choose. We install the kitchen sink, bathroom fixtures, toilet and shower at flow rates that meet current code, and add shutoffs so the unit can be serviced without turning off water to the main house. The final inspection covers all of it, and the unit is handed over with its shutoffs labeled.
What is included
What ADU plumbing includes
- Camera check of the existing lateral
- Sewer route and fall laid out
- Drain and vent rough-in
- Water and gas demand for both homes
- Line upsizing where needed
- Water heater for the unit
- Fixtures and shutoffs installed
- Inspections at rough-in and final
What happens after you call
How an ADU plumbing job runs
- 1
Site visit
Lateral, lines and layout reviewed.
- 2
Camera
Existing lateral checked before tie-in.
- 3
Plan
Sewer route and line sizes set.
- 4
Rough-in
Drains, vents and supply in place.
- 5
Inspection
City inspects before walls close.
- 6
Finish
Fixtures, water heater and final test.
Local context
San Jose's ADU boom and the permits behind it
The City of San José reports roughly 1,500 ADUs permitted since 2022 and more than 1,100 finaled, and says ADU permits have doubled in five years. Sewer lateral work tied to an ADU also needs a city plumbing permit and inspection. We plan the plumbing around the city's ADU process from the start, so the sewer connection, water and gas are inspected in the right order and the final is not held up by a line that was never permitted.
FAQ
ADU plumbing questions
Often, if the lateral is sound and sized for both.
Usually yes, placed near its fixtures.
Often; we handle the plumbing side of a separate meter.
A pump system may be needed; we check before design.
Yes; we plan the plumbing with your designer and contractor.
Yes, and inspected before walls and floors close.
Yes, including cutting the slab for new drains.
Related work
Other Kitchen & Bath jobs we handle
Guide: What ADU plumbing takes.
Planning an ADU in San Jose?
Call any hour; we plan the plumbing before the foundation.
Updated September 2026