How do Boise ADU utility connections work?
Boise allows ADUs to share the main house's sewer, water, and electrical services in most cases. Sharing is the default — the ADU runs off existing services with sub-metering for billing clarity on rentals. Separate services are an option if you want independent metering, but they add $5-15k in trenching, meter setters, and connection fees and rarely pay for themselves. See our overall ADU cost guide for utility line-item budgeting.
Sewer connections
Most Boise lots can share the main house's sewer lateral if it has capacity. The City of Boise's sewer card shows lateral location, depth, and condition. New laterals run $4-10k including trenching, pipe, and city inspection fees. If the existing lateral is in poor condition or undersized, the ADU build is the natural moment to replace it — the trench is already open.
Water service
Most Boise lots have a 3/4-inch water service serving the main house. Adding an ADU's fixtures (1 bath + 1 kitchen typically) usually fits inside that service's capacity. Lots with multiple bathrooms or premium fixtures may need an upsize to 1-inch service — $3-6k including the meter setter and connection fee through the City of Boise's water division. We pull the existing service size at the lot check.
Electrical service
A 200A main panel on the primary residence usually has enough headroom for a 100A ADU subpanel. Older 100A or 125A main panels need an upgrade to 200A, typically $2-5k including the panel, weatherhead, and ground rod. If Idaho Power's transformer feeding your address can't handle the additional load, they require a transformer upgrade at permit — typically $3-10k+ depending on whether trenching is required.
How to tell if you'll need a transformer upgrade
Idaho Power assesses each new ADU service request at permit. The trigger is whether the transformer feeding your address has capacity for the additional load. Older neighborhoods with 25kVA transformers serving 4-8 homes are the most likely to require an upgrade. Newer developments with 50kVA+ transformers per cluster usually pass without upgrade. We can call Idaho Power for a preliminary read during the lot check if it's relevant.
Gas service
Gas runs add trenching cost. Many new Boise ADUs go all-electric (mini-split heat, induction range, heat pump water heater) to skip the gas extension entirely and meet energy code targets more easily. If you want gas for cooking or a backup heat source, Intermountain Gas handles new service from the existing line; budget $1-3k for the extension and meter.
Submetering for rentals
Most Boise ADU landlords run submetering rather than separate services. The setup: utilities stay in the homeowner's name on shared services; submeters track ADU usage; the tenant pays a flat utility charge or actual-cost reimbursement based on submeter readings. Cheaper to install ($300-$800 per submeter vs $5-15k for separate services), simpler to operate, and easier to switch back to shared use if the rental relationship changes.
Common utility-cost surprises
- Idaho Power transformer upgrade — $3-10k+ when triggered
- Septic system replacement on rural lots — $15-30k
- Sewer lateral in poor condition discovered at trenching — $4-10k
- Long utility runs from the meter (>50 ft) — adds $1-3k per service
- Water main upsize from 3/4-inch to 1-inch — $3-6k
- Electrical panel upgrade from 100A or 125A to 200A — $2-5k
What we verify during the lot check
- Sewer vs septic (deal-breaker if septic and no upgrade space)
- Sewer lateral location and condition (sewer card if available)
- Existing water service size
- Main electrical panel capacity
- Idaho Power feed and likely transformer capacity
- Gas service availability if needed
- Distance from existing services to likely ADU location (drives trenching cost)
Where to next
Utilities are one of eight feasibility factors — the rest live in the ADU feasibility framework. Once utilities clear, see Boise's six pre-approved plans for buildable options or the design-build contractor overview for project structure. The free lot check runs the sewer/water/electrical assessment on your specific parcel.