Can a garage conversion use existing electrical?

Almost never as-is. A garage's existing electrical is designed for non-habitable, non-residential use and rarely meets residential code for a dwelling, so the existing wiring usually has to be evaluated and largely replaced, with a new load-calculated subpanel and circuit layout documented on permit drawings.

What a garage actually has vs. what an ADU needs

A standard California garage typically has just a few 15- or 20-amp general-purpose circuits for outlets and lights, plus a door-opener circuit — and it's often fed from the main house by a small sub-feed (sometimes 60 amps or less). An independent dwelling needs far more:

• Two 20-amp small-appliance circuits in the kitchen
• Dedicated circuits for the dishwasher, refrigerator, HVAC, and washer/dryer
• Bathroom circuits with GFCI protection
• Bedroom and living-area circuits with AFCI protection
• Smoke and CO detectors on the required circuits
• EV-ready conduit and a reserved breaker
• Code-compliant outlet spacing — no point more than 6 feet from an outlet along a wall, including the new wall where the door used to be

Two things that must be analyzed before reusing anything

1. The feed and panel capacity — the existing sub-feed to the garage is frequently too small for an ADU. A formal load calculation per NEC Article 220 determines the required subpanel size, and if the main house has a 100-amp service, that calculation often shows it can't support both the house and the ADU without an upgrade (panel upgrades commonly run $3,500-$7,000 and require utility coordination).
2. Existing outlet spacing and condition — garage outlets are not placed to residential spacing standards and the door-opener circuit becomes obsolete; both must be addressed on the plans rather than accepted as-is.

What can sometimes be reused

The existing conduit path from the main panel, the meter, or sometimes the sub-feed run may be reusable if the load calculation confirms adequate capacity — but that's a determination the engineer makes from the numbers, not an assumption. Everything downstream is typically re-designed to residential code.

Assuming the existing garage sub-feed is adequate is one of the most common causes of plan-check corrections and field conflicts on conversions.

The service-upgrade question

The biggest cost variable is whether the existing service can carry the new ADU. If the main house has a 200-amp panel with open slots, a load calculation may confirm there's room for an ADU subpanel and no upgrade is needed. If it's a 100-amp service, or a 200-amp panel already near full load, the calculation often points to a service upgrade or even a separate utility connection. This matters for your schedule as much as your budget: a panel upgrade requires the utility to coordinate a power shutoff at the meter, and that utility timeline runs separately from — and often longer than — the permit itself. Identifying it early in the plan stage keeps it from becoming a surprise that stalls the project near completion.

EV-ready and detector requirements people forget

Two items routinely missed on conversions are EV-readiness and life-safety devices. California requires new ADU electrical to include EV-ready conduit and a reserved breaker, and the plans must show it — retrofitting after walls close is far more expensive than including it now. Smoke and CO detectors must also be located on the floor plan and circuited correctly. Both are common, easily avoidable corrections.

Requirements vary by jurisdiction — confirm with your local building department. Our electrical plans include the load calculation, panel schedule, and GFCI/AFCI and EV-ready notation reviewers expect; you can start your order with your architectural plans and existing panel size.