Plumbing for San Jose ADUs and garage conversions
An ADU is a small house of its own, so the plumbing starts with how it connects to the sewer, water and gas that already serve the main house.
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ADU plumbing in San Jose covers the sewer connection from the new unit to the existing lateral or a new one, water and gas service sized for two households, and the rough-in for the unit's kitchen and bathroom; the work is permitted and inspected, and PG&E now requires a separate meter for ADUs.
Updated September 2026
Repipe, Gas & Water Quality
The plumbing questions every ADU starts with
Three questions shape the cost of ADU plumbing. Can the unit's drains reach the existing sewer lateral by gravity, or does it need a pump? Is the water service large enough to supply two kitchens and two bathrooms at once? And will the unit have its own gas service, a shared line or none at all?
We answer those from the site plan before the design is final, because moving the unit a few feet or changing the fixture layout can change the plumbing cost considerably.
Cost factors
What moves the cost of ADU plumbing
We have no published San Jose figure for ADU plumbing we trust, so the site plan sets the scope.
- Distance to the lateral. longer trench runs cost more
- Gravity or pump. an ejector pump adds equipment
- Existing lateral condition. a failing lateral needs repair first
- Water service size. upsizing for two households
- Gas or all-electric. gas piping or a heat pump heater
- Slab work. garage conversions need the floor cut
Included
What our ADU plumbing covers
- Site plan review for sewer, water and gas
- Camera inspection of the existing lateral
- Sewer tie-in or new lateral
- Water service sizing and branch
- Gas piping to the new meter
- Kitchen and bathroom rough-in
- Underground and rough-in inspections
Sewer tie-in or new lateral
Most backyard ADUs connect into the main house's existing lateral through a new wye and cleanout, provided the lateral is sound and the unit sits high enough for gravity drainage. We camera the existing lateral first, because tying a new unit into a cracked clay line only moves the problem. Where the grade does not allow gravity flow, a sewage ejector pump is the alternative. We lay out the trench route, depth and slope before the foundation is poured, because changing them afterwards means breaking new concrete.
Water and gas service to the unit
Two households drawing water at once can overwhelm a small service line. We calculate the combined fixture demand, check the meter and service line size, and run a new branch or upsize where needed, with a separate shutoff for the unit. Gas service follows the same logic: PG&E now requires ADUs to have their own utility meter, so we coordinate the gas piping with the new meter location and size it for the unit's appliances, or plan an all-electric unit with a heat pump water heater instead.
Garage and junior ADU conversions
Converting a garage or part of the house into a unit usually means breaking the slab for a bathroom and kitchen drain, since garages rarely have drains in the right place. We locate the existing sewer line, plan the shortest drain runs, and set the new drains and vents before the floor is patched. Junior ADUs inside the house's existing footprint often tie into nearby drains, which keeps the plumbing simpler, less disruptive and faster to inspect, and usually avoids cutting as much concrete.
Local context
San Jose's ADU boom and what it means for plumbing
San Jose issued just 10 ADU building permits in 2015; the city now reports 1,500 ADUs permitted since 2022 and more than 1,100 final building permits issued. PG&E requires ADUs to have their own utility meter, and the city's fire plan review can ask for a water company letter showing hydrant flow, with 1,000 gallons per minute at 20 psi as the benchmark. We raise both early in the design, since either can shift where the unit's utilities have to run.
How the job runs
How ADU plumbing runs
- 1
Review the site plan
Sewer, water and gas routes.
- 2
Camera the lateral
Before tying anything in.
- 3
Size the services
Water and gas for two households.
- 4
Underground rough-in
Sewer, water and gas trenches.
- 5
Unit rough-in
Kitchen and bathroom drains and supplies.
- 6
Inspect and finish
Underground and rough-in inspections, then fixtures.
Timing
How the plumbing fits the build schedule
Underground work usually takes a few days and must pass inspection before the foundation or slab is poured.
Rough-in inside the unit follows framing, and the finish plumbing comes after drywall. The city's plan review typically takes about 20 business days for the first round, so early plumbing planning keeps the design moving.
FAQ
ADU and Garage Conversion Plumbing questions from San Jose homeowners
Usually, if the lateral is sound and the unit drains by gravity. We camera the lateral first.
PG&E now requires ADUs to have their own utility meter. We coordinate the piping with it.
We calculate the combined fixture demand and upsize if needed.
Yes, usually by cutting the slab for new drains tied into the existing sewer.
Yes. We handle the plumbing scope and coordinate inspections with the builder.
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From our guides: Our adu plumbing guide.
One number, any hour
Plan the ADU plumbing early
Call with your site plan. We will map the sewer, water and gas routes before the design is final.
