Do ADUs Need Solar in California?
Generally yes for new detached ADUs. California's Energy Code requires solar PV on newly constructed detached ADUs, similar to the requirement for new single-family homes. Attached ADUs and garage conversions are typically exempt.
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What the energy code requires
Under the current California Energy Code, a newly built detached ADU must include a photovoltaic system sized to offset its modeled electricity demand, as part of the Title 24 compliance package. The system size comes out of the energy model, not a fixed kilowatt rule, so it scales with the ADU's size and efficiency features. The solar requirement is designed in alongside insulation, windows, and HVAC.
What is exempt
ADUs created within the existing home, additions that qualify as attached ADUs, and most garage conversions do not trigger the new-construction solar requirement. Shaded lots and certain small systems can qualify for exceptions, but these are narrowly defined. When in doubt, the Title 24 consultant's determination at the permit stage is what counts.
Batteries and practical notes
Battery storage is generally not required for ADUs the way it is incentivized for larger homes. Roof orientation and shading on a small ADU roof deserve early attention, since a detached ADU has limited roof area to work with. Coordinate the PV layout with the roof structural design so panel loads and attachment points are in the plans.
The bottom line
If you are building a new detached ADU in California, budget for rooftop solar. It is part of the energy code, not an optional add-on.