Planning Your Project
Does my ADU or addition need solar panels?
“California requires solar on ADUs” is too broad, and believing it has cost people real money on projects that never needed a panel.
It turns on classification, not on attached versus detached. Title 24 Part 6 §150.1(c)(14) requires a solar PV system on newly constructed single-family residential buildings, and a new detached ADU generally falls in that bucket. An attached ADU is treated as an addition, and a garage or interior conversion is an alteration — neither triggers the new-construction PV requirement. JADUs are exempt. There are also exceptions for very small calculated system sizes and for roofs without usable solar access.
The four buckets
| Project | Classification | Solar PV? |
|---|---|---|
| New detached ADU, built from the ground up | New construction | Generally required, unless an exception applies |
| Attached ADU built onto the existing house | Addition | Generally not required |
| Garage conversion, or conversion of existing conditioned space | Alteration | Not required |
| Junior ADU inside the existing walls | Alteration | Not required |
| Room or second-storey addition to your house | Addition | Generally not required |
| Garage fully demolished and rebuilt as a unit | New construction | Back in the required bucket — demolition changes the answer |
It is not attached versus detached. It is new construction versus addition versus alteration. That is why a detached garage CONVERSION is exempt while a detached new BUILD is not — both are detached, and they land in different buckets. If you demolish the garage rather than converting it, you have moved yourself into the requirement.
The exceptions that apply even to a new detached unit
- The calculated system is very small. Required PV is sized by formula against conditioned floor area and climate zone. Where the calculation lands below the code's small-system threshold, the requirement falls away automatically. Small units reach this more often than people expect.
- The roof has no usable solar access. Where permanent shading from trees, adjacent structures or a hillside leaves too little contiguous viable roof area, the requirement can be waived. The shading has to be from fixed external obstructions and it has to be documented in the permit application.
- The panels do not have to sit on the ADU. Where a system is required, it can generally be satisfied by installing on the primary dwelling instead. On a shaded backyard unit with a sunny main roof, that is often the better engineering answer as well as the compliant one.
Who actually decides this on your project
Not us, and not your solar salesperson. The determination comes out of the Title 24 energy compliance calculation, produced by the energy consultant as part of your permit set. That document sizes the system, or documents the exception, and it is what the city reviews.
We are a builder, not a design firm — but that does not mean you are on your own for this. We coordinate the architect, the structural engineer and the Title 24 consultant who produce your stamped permit set, and that work is a line item we price and manage. What we will not do is tell you a kilowatt number for your specific unit before the calculation has been run, because that number is an output of the calculation and not an opinion.
Why this belongs in the budget conversation early
Solar is frequently the single largest surprise line on an ADU budget, and it is a line that a quote can silently omit or silently include. If you are comparing bids on a new detached unit, ask each builder directly whether their number carries a PV system and what assumption they made about the exceptions. Two bids that answered that question differently are not comparable, and the gap is large enough to change which builder looks cheapest.
It is also worth knowing before the design is settled, because the choice between converting a garage and demolishing it moves you between buckets. That decision is usually made for reasons of layout, and the energy code consequence rides along with it unnoticed.
Common questions
Do ADUs require solar panels in California?
Does a garage conversion ADU need solar?
Does a home addition need solar panels?
Can the solar panels go on my main house instead of the ADU?
Who decides whether my project needs solar?
Where to go next
Written by the team at Elusive Construction, a licensed California general contractor (B #1112902) building in Santa Clara and San Mateo counties. General information about how residential construction works — not legal advice. For your specific contract, talk to an attorney.
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