Can my house's existing solar panels cover my new ADU's solar requirement?

No. If your ADU is a newly constructed detached unit, the California Energy Commission says an existing PV system on the main house cannot be used to meet the ADU's solar requirement, so the ADU needs newly installed PV unless an exception applies. You do have flexibility on where those new panels go, including adding new modules to the existing array.

What counts and what doesn't

• Does not count: panels that are already on the main house. Section 150.1(c)14 requires a newly installed PV system or newly installed PV modules, so capacity that already exists doesn't satisfy the new building.
• Can count: new modules added to the existing system. The CEC's 2025 ADU FAQ says new modules can be added to an existing PV system, and they don't have to sit on the ADU itself. They must be part of the ADU's permit application, sized to the Energy Code, and meet other building code requirements.
• Separate meter: if the detached ADU has its own meter, the CEC says new modules can still be added to the main house's system even if they won't serve the ADU. Conditions: the modules are on the same lot, and your load-serving entity (the utility) allows the system to be expanded.

When no new PV is needed at all

Some new ADUs end up with no PV requirement because an exception in Section 150.1(c)14 applies. The most common one for small ADUs: no PV is required when the calculated minimum system size is under 1.8 kWdc. Another: on steep-sloped roofs, no PV is required if the solar access roof area is less than 80 contiguous square feet. These are calculated in the Title 24 report, not assumed.

When none of this applies

The PV requirement only applies to newly constructed ADUs. The code says PV is not required for additions, so an attached ADU or a garage conversion does not need solar under the Energy Code. Local rules can add requirements, so check with your city.

Practical points before you call your solar installer

• Get the required size from the Title 24 report first, then design the array to match.
• If you plan to add modules to the existing system, confirm with your utility that the expansion is allowed under your interconnection agreement.
• Make sure the new modules appear in the ADU permit set, not a separate later permit, or the ADU may not get credit for them.

What this means for your MEP plans and Title 24 report

The CF1R records either the required PV size or the specific exception used. The electrical plans then need to show where the new PV ties in, whether that is a new array on the ADU or new modules on the existing system, and how it relates to the ADU's panel or meter. We coordinate both so they tell the same story at plan check. See the ADU Title 24 guide, how solar is shown on ADU electrical plans, and Title 24 reports.

Sources: energy.ca.gov, energy.ca.gov, energy.ca.gov.