Electrical work is one of the most misunderstood aspects of ADU construction. Homeowners often underestimate what is involved — from panel capacity and separate metering to the specific circuit requirements for modern building codes. Getting electrical right from the start prevents costly change orders, inspection failures, and delays in your Puget Sound Energy service connection.
This guide covers every electrical consideration for ADU construction in Bellingham and Whatcom County: whether you need a separate meter, what panel size your ADU requires, the specific circuits and safety features mandated by current code, and realistic costs from panel upgrades through final inspection.
For full construction costs including electrical, see our Whatcom County ADU cost guide. To understand how electrical fits into the overall build timeline, review our construction process.
Separate Meter vs. Shared Meter
One of the first electrical decisions for your ADU is whether to install a separate electric meter or share the existing meter with your main home. This decision affects costs, utility billing, and how you manage the ADU as a rental property.
Puget Sound Energy (PSE) requirements: PSE generally requires a separate meter for any dwelling unit that will be independently occupied. If you plan to rent your ADU, a separate meter is almost always required. If the ADU is for family use (aging parent suite, home office with kitchenette), a shared meter arrangement with a sub-panel may be acceptable.
Separate Meter
- Tenant pays their own electric bill directly to PSE
- Clean separation of utility costs for tax purposes
- Required for most rental ADU situations
- Cost: $1,500–$3,000 for meter base, pedestal, and PSE connection
Shared Meter with Sub-Panel
- Lower upfront cost — no separate meter installation
- Single electric bill for the property
- You must include electricity in rent or split with tenant
- May not be allowed for rental ADUs by PSE
ADU Panel Sizing
The electrical panel is the heart of your ADU's electrical system. It distributes power to every circuit in the unit and must be sized to handle the total electrical load. Undersizing the panel means you cannot add circuits later; oversizing adds unnecessary cost.
100-amp panel: Sufficient for most ADUs up to 1,000 square feet with gas appliances or a mini-split heat pump, electric water heater, standard kitchen appliances, and general lighting and outlets. This is the minimum panel size for a standalone dwelling and covers the majority of ADU projects in our area.
200-amp panel: Recommended for larger ADUs, all-electric homes with electric heating and cooking, properties planning for EV charging, or ADUs designed for future solar panel installation. If your ADU will have electric radiant floor heating, a large electric range, and an electric dryer, a 200-amp panel provides the headroom you need.
Your main home's panel matters too: If your main home has a 100-amp panel (common in older Bellingham homes built before 1980), you may need to upgrade the main panel to 200-amp service before adding the ADU load. Many homes in the Lettered Streets, Sehome, and Columbia neighborhoods have aging panels that need upgrading. This is one of the most common unexpected costs in ADU projects.
Required Circuits for an ADU
Current electrical code (National Electrical Code, adopted by Washington State) specifies minimum circuit requirements for any dwelling unit. Your ADU must have dedicated circuits for specific rooms and appliances. Here is what a typical ADU electrical layout includes:
Kitchen Circuits
Two dedicated 20-amp small appliance circuits for countertop receptacles. One dedicated circuit for the refrigerator (recommended though not always required by code). One dedicated circuit for the dishwasher. One dedicated 40-50 amp circuit for an electric range if applicable.
Bathroom Circuit
One dedicated 20-amp GFCI-protected circuit for the bathroom. If the ADU has two bathrooms, each needs its own circuit. All bathroom receptacles must be GFCI protected.
Laundry Circuit
One dedicated 20-amp circuit for the washing machine. If an electric dryer is planned, an additional 30-amp 240V circuit is required. Laundry receptacles must be GFCI protected per current code.
HVAC Dedicated Circuit
Mini-split heat pumps require a dedicated 30–40 amp, 240V circuit depending on the unit size. The outdoor compressor and indoor air handler each need power. This is one of the largest single loads in a typical ADU.
General Lighting & Receptacles
Multiple 15-amp or 20-amp circuits for general lighting and wall outlets throughout the ADU. The number depends on square footage and room layout. Code requires receptacles every 12 feet along walls and within 6 feet of any doorway.
Water Heater
Electric tank water heaters need a dedicated 30-amp 240V circuit. Heat pump water heaters (more energy efficient) require a dedicated 20–30 amp circuit. Tankless electric water heaters may need 40–60 amps at 240V.
Safety & Code Requirements
Modern electrical code includes several safety features that are non-negotiable for new construction, including ADUs. These requirements protect occupants and are verified during the electrical inspection before your ADU can receive an occupancy permit.
Arc-Fault Protection (AFCI)
AFCI breakers are required on all 15-amp and 20-amp circuits in bedrooms, living rooms, hallways, closets, and most other living spaces. They detect dangerous electrical arcs that can cause fires, providing protection beyond what standard breakers offer. AFCI breakers cost more than standard breakers but are mandatory for new construction.
Ground-Fault Protection (GFCI)
GFCI protection is required for all receptacles in kitchens, bathrooms, laundry areas, garages, outdoors, and within 6 feet of any water source. GFCI devices detect current leakage that could cause electrocution and cut power in milliseconds. They can be GFCI breakers in the panel or GFCI receptacles at each location.
Smoke & CO Detectors
Hard-wired, interconnected smoke detectors are required in every bedroom, outside each sleeping area, and on every level of the ADU. Carbon monoxide detectors are required outside sleeping areas. All detectors must be interconnected so that when one sounds, they all sound. Battery backup is required.
Emergency Egress & Exterior Lighting
Code requires exterior lighting at all entry and exit points. Emergency egress windows in bedrooms must have adequate clearance and may need dedicated lighting. Outdoor receptacles must be weather-rated and GFCI protected. At least one exterior receptacle is required at the front and back of the ADU.
Mini-Split Heat Pump Electrical Requirements
Mini-split heat pumps are the standard HVAC solution for ADUs in the Pacific Northwest. They provide both heating and cooling, are highly energy efficient, and require no ductwork. However, they have specific electrical requirements that must be accounted for in your ADU's electrical design.
Circuit requirements: A typical single-zone mini-split (9,000–18,000 BTU, suitable for most ADUs up to 750 square feet) requires a dedicated 30-amp, 240V circuit. Larger multi-zone systems for bigger ADUs may require a 40-amp circuit. The circuit must be dedicated — no other loads can share it.
Disconnect switch: Code requires an exterior disconnect switch within sight of the outdoor condenser unit. This allows service technicians to safely shut off power during maintenance. The disconnect must be accessible and clearly labeled.
Conduit and wire routing: The electrical connection between the indoor and outdoor units runs through a dedicated conduit, typically along the exterior wall. This also carries the refrigerant lines and condensate drain. Proper planning during framing ensures a clean installation without awkward routing.
For more on heat pumps and other sustainable building features, see our sustainable ADU construction guide.
Solar-Ready Requirements
Some jurisdictions in Washington State now require new construction to be “solar-ready,” meaning that even if you do not install solar panels immediately, the infrastructure must be in place for a future installation. Even where not strictly required, building solar-ready is inexpensive during construction and extremely expensive to retrofit later.
What solar-ready means in practice: A conduit pathway from the roof to the electrical panel area (typically 1-inch EMT conduit), reserved space in the electrical panel for a solar breaker (two-pole 30-amp or 40-amp), a designated area near the panel for a future inverter, and roof structural capacity for panel mounting. During new construction, these preparations add only $200–$500 to the electrical cost.
Economics of ADU solar: A south-facing ADU roof can typically accommodate a 3–5 kW solar array, generating approximately 3,500–5,500 kWh annually in Bellingham. At current PSE rates, that offsets $400–$650/year in electric costs. With the 30% federal Residential Clean Energy Credit and PSE net metering, the payback period is typically 8–12 years. The panels add value to the property well beyond their cost.
ADU Electrical Cost Breakdown
| Item | Cost Range |
|---|---|
| Main home panel upgrade (100A to 200A) | $2,000–$5,000 |
| Separate meter installation (meter base, pedestal, PSE fees) | $1,500–$3,000 |
| ADU panel (100A or 200A with breakers) | $1,500–$3,000 |
| Interior wiring (all circuits, receptacles, switches) | $4,000–$8,000 |
| Lighting fixtures and installation | $1,000–$3,000 |
| Smoke/CO detectors (hard-wired, interconnected) | $300–$600 |
| Heat pump dedicated circuit and disconnect | $500–$1,000 |
| Exterior receptacles and lighting | $400–$800 |
| Permits and inspections | $300–$600 |
| Total electrical (without main panel upgrade) | $8,000–$15,000 |
| Total electrical (with main panel upgrade) | $12,000–$25,000 |
*Cost estimates for Bellingham and Whatcom County as of 2025–2026. Actual costs depend on ADU size, panel requirements, meter configuration, and fixture selections. All costs are part of overall ADU construction costs.
PSE Coordination Timeline
Puget Sound Energy is your electric utility provider, and their involvement is required for any new service connection or significant service upgrade. Planning for PSE coordination is critical because their timelines are outside your control and can create bottlenecks if not managed proactively.
Service Application (Week 1–2)
Submit a new service or service upgrade application to PSE. This should happen as early as possible in your project — ideally during the permitting phase, not after construction starts.
Engineering Review (Week 2–6)
PSE engineers review your service request and determine what transformer and line work may be needed. If a transformer upgrade is required, this can add 4–8 additional weeks.
Meter Installation (Week 6–10)
After your electrician installs the meter base and it passes inspection, PSE schedules the meter set. Current wait times for meter installation are typically 2–4 weeks after passing inspection.
Service Energized (Week 8–12)
Once the meter is set and all inspections pass, PSE energizes the service. Your ADU now has permanent power. Temporary construction power can be arranged earlier in the process if needed.
We initiate PSE coordination during the design phase to ensure electrical service is ready when your ADU construction reaches that stage. This proactive approach prevents the delays that catch many first-time ADU builders by surprise.
Frequently Asked Questions
Does my ADU need its own electrical meter?
It depends on your situation and local requirements. Puget Sound Energy (PSE) generally requires a separate meter for an ADU if it will be rented out as a separate dwelling unit. A separate meter allows independent utility billing and simplifies landlord-tenant arrangements. If the ADU is for family use and you prefer a single electric bill, a shared meter with a sub-panel may be acceptable. We coordinate with PSE early in the design process to confirm requirements for your specific project.
How much does it cost to upgrade my main panel for an ADU?
A main panel upgrade from 100-amp to 200-amp service typically costs $2,000 to $5,000 including the panel, breakers, labor, and PSE coordination. If your home still has a Federal Pacific, Zinsco, or fuse-type panel, the upgrade is especially important for safety and is often required by insurance. Many older Bellingham homes built before 1990 have 100-amp or even 60-amp panels that cannot support the additional load of an ADU without upgrading.
Can I add solar panels to my ADU?
Yes, and some jurisdictions may require solar-ready conduit even if you do not install panels immediately. A south-facing ADU roof is ideal for solar production. The electrical system should be designed with solar in mind from the start, including space in the panel for a solar breaker, conduit from the roof to the panel area, and a potential battery storage location. The federal 30% Residential Clean Energy Credit applies to solar installations, making the economics increasingly attractive. Our sustainable construction approach incorporates solar-ready design into every ADU.
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