Detached ADU Title 24 Report
A new detached ADU is a newly constructed building under the Energy Code, so it is documented the way a new single-family home is: the full prescriptive package or a performance model, newly installed solar PV unless an exception applies, and ECC-Rater testing of the measures that need it. Being small does not move it onto the lighter rules for additions.
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Why a backyard unit is treated as a new house
The Energy Code has no ADU category. The Energy Commission’s ADU guidance treats a new structure standing apart from the main house as a newly constructed building, one never used or occupied for any purpose. The compliance manual spells out the consequence: for all intents and purposes the unit is a new single-family home, held to Section 150.0’s mandatory measures and Section 150.1’s prescriptive or performance standards.
A covered breezeway to the house does not change that; the Commission’s examples show a newly constructed ADU with an optional breezeway. A shared wall, floor or ceiling does, making it an attached ADU. A unit over a garage can go either way and has its own page. The 2025 Energy Code applies to permit applications submitted on or after 1 January 2026.
Prescriptive package or performance model
On the prescriptive path the unit meets the component package in Table 150.1-A for its climate zone, documented on form CF1R-NCB-01-E. On the performance path, Commission-certified software compares the design with a standard design built to that package, measured in source energy and long-term system cost, and issues a CF1R-PRF-01-E.
Glazing is where small units tend to leave the prescriptive path. The package caps window and glazed-door area at 20 per cent of the conditioned floor area in every zone, so a 450 square foot unit is allowed 90 square feet. A wall of glass doors can exceed that alone, and the performance model is then the route.
What the new-home package asks of a small unit
- A heat pump for space heating under Section 150.1(c)6, or a performance model that shows another system complies.
- A 240-volt heat pump water heater, with two exceptions: a unit of 500 square feet or less may use an electric water heater with point-of-use distribution, and one with one bedroom or less may use a 120-volt heat pump water heater.
- Quality Insulation Installation in all 16 climate zones.
- Whole-dwelling mechanical ventilation whatever the floor area, with the airflow field verified, and a vented range hood whose rating is verified.
- Battery-ready equipment under Section 150.0(s) when the electric service exceeds 125 amps. A 200 amp panel can meet the panel option only if its busbar is rated and marked 225 amps.
- Electric-ready circuits for any gas water heater, furnace, cooktop or dryer.
Size does help in places. A unit of 500 square feet or less needs no whole-house fan, and in climate zone 5 may use windows of U-0.30 rather than 0.27. Solar-ready roof area is not required at all, because Section 110.10 covers subdivisions of ten or more homes, not a new unit on an existing lot.
Solar and on-site testing
The unit needs newly installed PV sized under Section 150.1(c)14 unless an exception applies. The main house’s existing array cannot be counted, but new modules added to it on the same lot can, if they are part of the ADU permit and the utility allows it. More on ADU solar.
An ECC-Rater (a HERS rater until the rename on 1 January 2026) then tests several measures on site: refrigerant charge on a heat pump in every zone, duct leakage and fan efficacy on a ducted system, minimum airflow, QII, and any SEER2, HSPF2 or heating capacity the model credits. Where testing is needed, the CF1R is registered with an ECC-Provider before submittal.
What to send for a detached unit
On top of the usual ADU drawings: a site plan with a north arrow and anything that shades the ADU roof, for the solar sizing; the bedroom count, which sets the ventilation rate; the electric service size and busbar rating; and whether the PV goes on the ADU or the main house.
Where detached submittals come back
Corrections specific to new detached units come from documenting them as something lighter than a new house: addition forms with no PV, solar credited to the existing array, glazing over the 20 per cent cap on a prescriptive form, a gas appliance without its electric-ready circuit, or a 200 amp panel offered as battery-ready with no 225 amp busbar marking. Each is cleared by reissuing the documents, as described under ADU plan check corrections.
Other ADU Types
Frequently Asked Questions
No. The Energy Commission’s ADU guidance shows a newly constructed ADU with an optional breezeway as new construction. A unit is treated as attached only when it shares a wall, floor or ceiling with another dwelling unit.
Only through an exception, such as a calculated PV size under 1.8 kWdc or, on a steep-sloped roof, a solar access roof area under 80 contiguous square feet. Otherwise PV is required unless the design complies without it on the performance path.
No. The Energy Code does not say where the new modules go or which meter they serve. They can be added to the main house’s system on the same lot if the utility allows the expansion.
From $199, including up to 1,000 square feet, plus $9 for each additional 100 square feet. Standard turnaround is 2–3 business days, a 24-hour rush adds $95, and revisions are unlimited until the project complies.