Electrical Planning for an All-Electric ADU in Marin

Maybe — it depends on your current service size and how much the main house already uses, and the only way to know for sure is a load calculation. An all-electric ADU adds meaningful demand: a heat pump for heating and cooling, induction cooking, electric water heating, and often EV charging. A load calc tallies that against your existing service to tell you objectively whether it fits or whether you need a service upgrade first. Building without that answer risks either an overloaded service or an unnecessary upgrade.

A load calculation is a code-based accounting of total electrical demand — here, the ADU’s heating, cooking, water heating, and charging loads added to your existing home — measured against your service capacity. It’s essential because an ADU is a significant new load, and guessing in either direction is costly: underestimate and you overload the service, overestimate and you pay for an upgrade you didn’t need. The load calc turns “will it fit?” into an objective yes, no, or upgrade-by-this-much answer that the whole electrical plan depends on.

Almost always, yes — a dedicated subpanel in the ADU, fed by a feeder from your main service or an upgraded one, is the standard approach. The subpanel distributes the ADU’s own circuits and keeps its electrical organized and separately manageable. Whether your existing main service can feed that subpanel directly, or needs upgrading first, is exactly what the load calculation determines. The subpanel and its correctly sized feeder are core parts of the ADU electrical design.

Sometimes, but not always — it comes down to the load calculation. If your existing service has enough spare capacity to carry the ADU’s added load, you may be able to feed the ADU subpanel without upgrading. If the combined demand exceeds what your service can safely deliver, a service upgrade becomes the foundation for the project. Many Marin homes with older, smaller services do need an upgrade to support an all-electric ADU, but the calc tells you definitively rather than assuming.

Often, yes. California and various utility and state programs periodically offer incentives for electrification and efficient equipment like heat pumps and heat-pump water heaters, and all-electric construction can qualify. Programs and amounts change, so it’s worth checking current offerings through state energy resources and your utility while planning. Designing the ADU all-electric from the start positions you to capture whatever incentives apply, whereas retrofitting to all-electric later is both costlier and may miss new-construction-oriented programs.

Because retrofitting electrical capacity into a finished ADU means opening walls, reworking panels, and sometimes upgrading the service after the fact — all far more expensive and disruptive than planning it up front. Deciding the service question, sizing the subpanel and feeder, and laying out circuits before construction lets everything be built once, correctly, and to code. An all-electric ADU is an efficient, future-proof addition when the electrical is planned from day one, and a source of costly surprises when it isn’t.