Does my existing main panel need to be upgraded to add an ADU?

Maybe — and the only reliable way to know is a formal NEC Article 220 load calculation that adds the new ADU load on top of your existing house load and checks the total against your main service. A panel or service upgrade is triggered when that combined demand exceeds what the existing main can safely carry, not by ADU square footage or a rule of thumb.

The common scenarios

An all-electric ADU pushes harder on the main than a gas-equipped one, because induction cooking, a heat pump water heater, heat pump HVAC, and EV charging all become electrical loads. That makes an upgrade more likely on older services.

What an upgrade involves

• Cost. Main service or panel upgrades commonly run in the range of $3,500–$7,000 depending on amperage, panel location, and whether the meter base and service entrance also change.
• Utility coordination. The utility must de-energize at the meter to perform the work, so a service upgrade typically carries 6–12 weeks of utility lead time — often longer than the building permit itself.
• Inspection. The new panel needs proper working clearance (roughly 30 inches wide by 36 inches deep of clear space in front) and updated grounding/bonding.

Alternatives when the main is maxed out

An upgrade isn't the only path. Depending on the calc and your utility's rules, options include:

• Upgrading the main service to 200A, 225A, or 320A so a subpanel can be added.
• A load-management device (such as a smart panel or an automatic load-shedding controller) that can let an existing service host the ADU without a full upgrade, where the jurisdiction accepts it.
• A separate utility service and meter for the ADU — sometimes required for distance or separate billing, and the right answer when the main truly can't be expanded.

Which option is viable is a calculation-and-utility question, not a preference; the load calc points the way. If an upgrade looks likely, open the utility request in parallel with permitting so the timelines overlap.

Requirements vary by jurisdiction and utility — confirm with your local building department and serving utility.

Every one of our electrical plan sets includes the load calculation that determines whether an upgrade is required and documents the feeder or service design either way. Start with how it works if you're unsure where your project stands.