Can Existing Solar Panels Count for a New ADU?
No. A new detached ADU needs newly installed PV under Section 150.1(c)14, and the Energy Commission says an existing system cannot be used to meet it. An owner who already has solar still has three real routes: a new array, new modules added to the house system, or an exception. The house's own array is not re-judged.
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What the code counts
The text of Section 150.1(c)14 asks for a "newly installed photovoltaic (PV) system or newly installed PV modules" meeting Joint Appendix JA11, which sets qualification rules for PV used to meet the standards. Kilowatts already on the house earn the ADU nothing, and the ADU's own calculated size stands in full.
The three routes
- A new array on the ADU. Solar access roof area includes the ADU roof and any carport or other newly constructed structure on the site that can carry PV, and the size is capped by that area times 18 W per square foot (14 on a low slope).
- New modules joined to the house system. The conditions and paperwork are on adding ADU panels to the main house system.
- An exception or a performance design that needs no PV, set out under ADU solar exceptions.
HCD's handbook confirms the panels can be on the ADU or on the primary dwelling.
What happens to the existing system
The ADU is a newly constructed building with its own compliance. Nothing in the alteration list in Section 150.2(b)1, which runs from windows through ventilation and exterior doors, concerns PV, so the old array is not retested because an ADU is built beside it. The existing-home solar page covers that side.
What to hand the building department
- A site plan showing the existing array and any new modules, and which structure carries them.
- The ADU's compliance form (CF1R-NCB-01-E, or CF1R-PRF-01-E on the performance path) listing the new PV size.
- A shading study from an approved solar assessment tool if an exception rests on limited solar access.
- After installation, the PV certificate of installation, CF2R-PVB-01-E.
The common mistake
Entering the house array's kW on the ADU forms as if it were credit. The related rules are on ADU solar and the detached ADU report; orders start on the ADU page.
Common Title 24 Questions
- Do ADUs need Title 24 in California?
- What plans are needed for an ADU Title 24 report?
- How much does an ADU Title 24 report cost?
- How long does an ADU Title 24 report take?
- Does a garage conversion ADU need Title 24?
- Does a JADU need a Title 24 report?
Frequently Asked Questions
No. The Energy Commission says a newly constructed detached ADU needs newly installed PV unless an exception applies, whatever the size of the existing system.
No. A conversion is an addition or an alteration, and Exception 6 to Section 150.2(a) states PV is not required for additions.
Explore ADU and JADU
- Can New ADU Solar Panels Be Added to the Main House System?
- Do ADUs Need Title 24 in California?
- Ducted vs Ductless Mini-Split for an ADU: Title 24 Differences
- Garage Conversion ADU Title 24 Checklist
- Does a Garage Conversion ADU Need Title 24?
View all ADU and JADU guides → · Permits and Compliance · Title 24 guide