Under the 2025 Energy Code, a new detached ADU is treated as a new home, an attached ADU or garage conversion is treated as an addition, and an ADU or JADU built inside existing heated or cooled space is treated as an alteration. That one label decides most of your requirements, including whether you need solar.
Everything below comes from the California Energy Commission's 2025 ADU FAQ and its 2025 Single-Family Compliance Manual. The 2025 code applies to permit applications submitted on or after January 1, 2026.
Is my ADU new construction, an addition or an alteration?
| Your ADU | How the Energy Code treats it | Main rules |
|---|---|---|
| New detached ADU | Newly constructed building | Same rules as a new single-family home |
| Attached ADU (shares a wall, floor or ceiling) | Addition | Section 150.2(a) |
| Garage or storage conversion (attached or detached) | Addition | Section 150.2(a) |
| Conversion of space that is already conditioned | Alteration | Section 150.2(b), for the parts you change |
| JADU | Depends on scope | Addition rules if it adds conditioned space; alteration rules if not |
The CEC defines an addition as any change that increases conditioned floor area and conditioned volume. That is why a garage conversion counts as an addition even though no new walls go up.
Does a new detached ADU need solar panels?
Usually, yes. The CEC says newly constructed detached ADUs "must have a newly installed solar PV" to meet Section 150.1(c)14, unless an exception applies. Your Title 24 report sizes the system.
A few details from the CEC FAQ:
- The main house's existing solar does not count. It must be new PV.
- New modules can be added to an existing system. They do not have to sit on the ADU. They must be on the same lot, be part of the ADU's permit, and the utility must allow the expansion.
- Separate meters are fine. The code does not say whether the new modules must serve the ADU meter or the main house meter.
- Solar-ready does not apply. Solar-ready rules cover new homes in subdivisions of ten or more, not an ADU on an existing lot.
Does an attached ADU or garage conversion need solar?
No. The CEC says attached ADUs are additions, "solar PV is not required." The compliance manual adds that PV requirements do not apply to additions or alterations. A garage conversion is an addition, so the same applies.
Does a detached ADU need to be battery-ready?
Yes, in most cases. The CEC says a newly constructed ADU must meet the battery-ready requirements in Section 150.0(s). Those requirements do not apply if the ADU's electric service is 125 amps or less. Installing a full battery system also satisfies them.
Can I use gas in a new detached ADU?
The CEC says a new detached ADU with gas water heating, space heating, cooktop or dryer must meet the electric-ready requirements in Sections 150.0(n) and 150.0(t) through (v). That means the wiring and space for a future electric appliance must be in place.
What about the water heater in a small ADU?
Small ADUs have a simpler option. Under the prescriptive rules:
- A new dwelling of 500 square feet or less may use an electric water heater with point-of-use distribution (Section 150.1(c)8, Exception 1).
- An ADU addition of 500 square feet or less may do the same (Section 150.2(a)1Diii).
Point-of-use means short pipe runs. The CEC gives an example: up to 15 feet of 3/8-inch pipe or 10 feet of 1/2-inch pipe, per Reference Appendix RA4.4.5.
Does my ADU need quality insulation inspection (QII)?
It depends on the type and size.
- ADU additions of 700 square feet or less: the prescriptive QII requirement does not apply (Section 150.2(a)1Bv). If the report takes QII as a credit, a rater must still verify it.
- Additions over 700 square feet: QII applies to the addition. For a converted garage, the manual says QII does not include header insulation or air sealing if the existing air barrier stays in place.
- New detached ADUs: these follow new-home rules.
Do ADUs need ventilation and a kitchen hood check?
- New detached ADUs: the CEC says every newly constructed dwelling must meet the indoor air quality ventilation rules in Section 150.0(o), no matter how small.
- Kitchen hoods: the CEC says vented range hoods in ADUs must have their certification, airflow and sone rating verified by an ECC-Rater.
- JADUs: ventilation may be required by the California Mechanical Code. The CEC points JADU ventilation questions to HCD.
Can I heat and cool my ADU with a mini-split?
Yes. The CEC confirms mini-splits and multi-splits can be used. They may trigger ECC-Rater field testing, including verified SEER2 and HSPF2 ratings, verified heating capacity, and refrigerant charge. The 2025 code requires refrigerant charge verification for heat pumps in all climate zones (Section 150.1(c)7A).
You cannot simply extend the main house's furnace ducts into an attached ADU. The CEC notes the California Mechanical Code does not allow return air from one dwelling to be sent into another.
Do ADU additions need a cool roof?
In some climate zones. Additions must meet cool roof rules for steep-sloped roofs in climate zones 10 through 15, and low-sloped roofs in zones 13 and 15. Additions of 300 square feet or less are exempt.
What about converting a garage with 2x6 walls?
If the conversion is an addition, the existing 2x6 walls can be insulated with R-21 in the cavity for prescriptive compliance. Continuous insulation is not required for those walls. The CEC also notes that existing walls around newly conditioned space count as "new." You cannot claim a credit for verifying them as existing.
What if part of the building was never permitted?
The CEC says unpermitted work must meet the Energy Code when you apply for a new permit. The building department decides whether the ADU complies as new construction or as an addition.
What does this mean for my report?
The label also changes how much your Title 24 report has to cover:
- A new detached ADU needs a full new-home analysis, including PV sizing.
- An attached ADU or garage conversion is analyzed as an addition. It can be the addition alone, or the existing house plus the addition.
- A conversion of conditioned space covers only the parts you change.
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Frequently asked questions
Does a detached ADU need solar panels under the 2025 Energy Code?
Usually yes. The CEC says a newly constructed detached ADU must have newly installed solar PV to meet Section 150.1(c)14, unless an exception applies. Using the main house's existing system does not count, but adding new modules to it on the same lot can.
Does an attached ADU need solar?
No. The CEC says attached ADUs are additions, and solar PV is not required for additions.
Is a garage conversion ADU a new building or an addition?
An addition. The CEC treats converting unconditioned space, like a garage, into conditioned space as an addition, whether the garage is attached or detached. It complies under Section 150.2(a).
Does a JADU need a Title 24 report?
It depends on the work. A JADU built inside existing conditioned space is an alteration, and Energy Code rules apply to the parts you change, such as a new water heater, mini-split or lighting. A JADU that adds conditioned space is part of an addition.
Does my ADU need quality insulation inspection (QII)?
Not for ADU additions of 700 square feet or less under the prescriptive rules. The exception does not apply if the report takes QII as a credit. New detached ADUs are new homes, so the new-home rules apply.
Can a small ADU use an electric water heater instead of a heat pump water heater?
Yes, in some cases. A new dwelling of 500 square feet or less, or an ADU addition of 500 square feet or less, may use an electric water heater with point-of-use distribution under the prescriptive rules.