An ADU is a whole second home, electrically

It is easy to think of an ADU as 'just a small addition.' Electrically, it is a second kitchen, a second HVAC system, a second water heater, and a second set of everything that draws current. On a 100A service that is already feeding the main house, an ADU does not fit — the math fails before the first wall goes up.

Load calculation comes before floor plans

Electrical guide illustrationIllustrative photo

This is the sequencing that saves people money: electrical first, ADU design second. A licensed electrician runs a load calculation combining the existing house and the planned ADU. The result tells your ADU designer the real constraint — how many amps are actually available — instead of discovering mid-construction that the panel cannot support the design.

Your three electrical paths

  1. Upgrade the main service to 200A (or larger). The most common answer. One upgraded service feeds both the main house and the ADU, usually through a subpanel in the ADU. Clean, single-meter, and sized for everything at once.
  2. Subpanel off the existing panel. Only works when the load calculation shows real spare capacity — rare on 100A services, occasionally possible on 200A ones. Cheapest when it works, but the numbers have to work first.
  3. Separate meter and service for the ADU. Sometimes required by the utility or preferred for rental accounting. More infrastructure, more permits, more cost — but fully independent.

The order that saves money

We have seen ADU projects stall for months because the panel question was asked last. Ask it first: one load calculation, one straight answer about service size, and your ADU designer works with real numbers from day one. The load calculation itself is a small, fixed-scope visit — a fraction of what a stalled ADU project costs per week in holding costs and rescheduled trades. Electrical is the cheapest part of the project to plan and the most expensive to fix late.

Planning an ADU? Send us three panel photos, your city, and the ADU size you have in mind. Free preliminary assessment — we will tell you which of the three paths your home is on.