Cost to Wire an Addition or ADU in Marin

The main drivers are the ADU’s size and electrical load — how many circuits it needs for appliances, HVAC, laundry, and any heavy loads like a heat pump or EV charger — plus the distance from your main panel, whether a subpanel is used, and critically, whether your existing service has enough capacity or needs upgrading. Permits and inspection are also part of the total. A compact ADU on a home with spare service capacity costs far less than one that forces a service upgrade, which is why the capacity question dominates the budget.

For a sizable addition or most ADUs, yes — a subpanel in the new space is usually more economical and practical than running many individual circuits all the way back to the main panel. It centralizes the new circuits locally and simplifies the wiring. Smaller additions with only a few circuits might tie into the existing panel if there’s room. Whether a subpanel makes sense depends on the number of circuits and the distance, which an electrician assesses when scoping the job.

It might. An ADU with a kitchen, laundry, HVAC, and possibly an EV charger adds significant load, and if your existing service doesn’t have enough spare capacity to carry it, a service upgrade becomes necessary before the ADU can be powered safely. This is one of the larger potential line items. A load calculation — totaling your current and new demand against your service size — tells you whether you can feed the ADU from your existing service or need to upgrade first.

Yes. Additions and ADUs are permitted, inspected construction projects, and the electrical work must be permitted and pass inspection to confirm it meets the current code and energy standards. Permit and inspection costs are a required part of the budget, not optional. Beyond compliance, permitted work protects you at resale and with your insurer — unpermitted electrical work on an ADU is a problem that surfaces later. A licensed contractor handles the permitting as part of the project.

It depends on your panel’s capacity and available space. If your main panel has spare capacity and room, it may feed a subpanel for the new space directly. If it’s full or your total service is maxed out, you may need a service upgrade or a larger panel first. This is exactly what a load calculation determines. Rather than assuming, have an electrician evaluate your panel and service against the ADU’s planned load before finalizing the design and budget.

Plan the loads realistically rather than over-provisioning, place the subpanel to minimize long runs, and — if you’re near your service limit — consider whether load management or right-sizing appliances can avoid a full service upgrade. Coordinating the electrical design early with the overall ADU plan also prevents costly changes mid-build. The single biggest saver is knowing your capacity situation up front via a load calculation, so you design within your service where possible and budget accurately where an upgrade is genuinely needed.