ADU · California Title 24

Does my ADU need solar?

Generally, yes for a new detached ADU — and an existing system on the main house usually doesn’t cover it. Here’s the actual rule and where the exceptions are.

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The short answer

A new detached ADU generally needs its own PV system

A newly constructed, detached ADU is generally treated as new construction under Title 24 — and new residential construction generally requires a newly installed solar PV system sized for that unit. An existing PV array on the main house doesn’t automatically satisfy the ADU’s requirement on its own — new modules can sometimes be added to an existing array, but they have to be sized and permitted specifically for the ADU, not just left as-is.

There are exceptions that come up often enough to be worth knowing before you assume the worst case:

No PV required if the calculated minimum system size works out to under 1.8 kWdc — common on a small ADU footprint.
No PV required if the available Solar Access Roof Area is under 80 contiguous square feet — heavy shading or a small/complex roof can trigger this.
JADUs and interior or attached ADUs aren’t newly constructed buildings — the code treats converted or added-on space as an addition (or, for space that was already conditioned, an alteration) — and the PV mandate applies only to new construction, so they’re generally exempt (Section 150.2(a), Exception 6).
A garage conversion to ADU is treated as an addition, not a newly constructed building — generally exempt from the mandate for the same reason.

Planning two detached units on one lot? That becomes allowed statewide on January 1, 2027 — see the new ADU laws for 2027 for what changes and how the solar math works per building.

“Generally” is doing real work in those sentences — solar sizing depends on the specific project, roof, and how the local building department interprets the exceptions. Send the plans and the model will tell you exactly where your project lands, with the assumption stated plainly, not buried.

Rates

Prices

Flat fees, in writing before I start. No hourly, no bid package, no change-order games — the number on the quote is the number on the invoice.

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Disclosure

One honest note

I’m not a licensed engineer or architect, and I don’t stamp drawings. On the CF1R I’m the documentation author — you stay the responsible designer. Nothing about your role on the job changes.

That’s on purpose. I’m not after your scope or your clients. I do one piece of the permit set — the piece that usually holds it up — and I do it fast.

Questions

ADU solar questions

If yours isn’t here, see the full FAQ or call and ask.

Does my ADU or addition actually need one?

Almost certainly. New conditioned floor area — ADUs, JADUs, garage conversions, additions — triggers Title 24. So do alterations that touch the envelope, the ductwork, or the HVAC equipment, including a straight change-out. A remodel that only moves non-structural interior partitions usually doesn’t.

Send it either way. If it doesn’t need a report I’ll say so and there’s no charge for the look.

Does a garage conversion ADU need solar?

Generally no — a garage conversion is treated by the Energy Code as an addition rather than a newly constructed building, which generally exempts it from the new-construction solar mandate that applies to a newly built detached ADU. See garage conversion vs. detached ADU for the fuller comparison.

Does a JADU need solar?

Generally no. A JADU sits inside the existing home, so the code treats it as an addition or alteration rather than a newly constructed building, which generally puts it outside the new-construction PV mandate that applies to detached ADUs.

Can I just add panels to my house's existing solar array instead of a separate ADU system?

Sometimes, but not automatically. An existing PV system on the main house doesn't by itself satisfy a new detached ADU's separate requirement — any added capacity has to be sized and permitted specifically for the ADU. Whether that's the right approach for your project depends on the array and the ADU's own energy budget — the model settles it.

Find out where your ADU lands.

Send the plans and the CF1R comes back with the solar requirement (or exemption) stated plainly, not buried in an appendix.

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