Does a new ADU need solar panels in California?

Here is the rule, from the California Energy Commission's guidance on the state energy code: a newly constructed DETACHED ADU must include a new solar electric system. A garage conversion does not. An attached ADU does not. The code treats conversions of existing space and attached additions as additions to the existing home, and additions carry no solar requirement.

For a detached new build, the details matter and mostly help you. The new panels do not have to sit on the ADU itself; they can be added to an existing solar system anywhere on your property, as long as they're new modules sized to the code and included in the ADU's permit. And there are exceptions: if the usable, unshaded roof area is too small, or the required system would be under 1.8 kilowatts, the requirement can fall away entirely. Plan check makes that call.

Budget-wise this is one of the larger line items that quotes love to leave out, which is why the free ADU budget on this site lists solar as a separate item on detached builds and tells you explicitly when your build type is exempt. If a bid for a detached new build doesn't mention solar at all, that's not a discount; it's a change order waiting for you at plan check.

You'll hear the wrong version of this rule in both directions: that every ADU needs solar (no), or that solar is optional if you just call the unit something else (also no; the code goes by what's physically being built). When a contractor gets this rule right unprompted, it's a good sign they've actually pulled ADU permits recently.