The four utility connections every ADU needs
Almost every California ADU project involves four utility connections:
- Water
- Sewer (or septic in rural areas)
- Electrical
- Gas (optional — many all-electric ADUs skip this)
Each has its own permit, inspection, and cost driver.
Water hookup
Most ADUs tap the existing water service to the main house. If the existing meter and service line are adequately sized, the ADU just adds a branch. If demand exceeds the existing service, the water utility may require a meter upgrade or a new service.
Cost drivers: Trenching distance, depth, whether the existing meter is adequate.
Sewer hookup
ADUs tie into the main house's existing sewer lateral in almost all cases. The new ADU's plumbing branches into the lateral upstream of the cleanout. Sewer connection fees are a common surprise — they vary widely by city and can be substantial.
Cost drivers: Distance from ADU to lateral, slope requirements, sewer connection fee.
Electrical hookup
This is the biggest variable. Three scenarios:
- Existing panel has capacity: A subpanel is added in the ADU and fed from the main panel. Lowest cost.
- Panel needs upgrade: Main service is upgraded (often from 100A to 200A). Mid-cost.
- New service required: Utility runs a separate service to the ADU. Highest cost; longest schedule because the utility controls timing.
Cost drivers: Existing panel capacity, distance from panel to ADU, whether trenching is required.
Gas hookup
Many California ADUs are designed all-electric, which skips gas entirely and aligns with state building decarbonization goals. If you want gas appliances:
- Branch off the existing main house gas line if capacity allows.
- Or run a new line from the utility (rare for ADUs).
Cost drivers: Distance from main line, whether existing capacity is adequate.
Independent metering: yes or no?
You can ask the utility to install separate meters for the ADU. This costs more upfront but lets the ADU occupant pay their own utility bills directly. Most rental ADUs benefit from independent metering; most family-use ADUs do not.
What your builder should do before quoting
A good builder inspects the main panel, locates the sewer lateral cleanout, and notes the water meter location during the site visit. If they quote utility hookups without doing these three things, the bid is a guess.
Related reading
- 1,200 sq ft ADU cost breakdown
- Garage conversion permits
- Detached vs attached ADU
- Vision ADU Service Areas
Frequently asked questions
Do I need separate utility meters for my ADU? Not required by state law, but often a good idea for rental units.
How long does an electrical service upgrade take? Permit and utility coordination usually adds 4-12 weeks.
Can my ADU be all-electric? Yes, and many new ADUs are. This also avoids gas line and meter costs.