ADU Attached to a Garage: How Title 24 Classifies the Project
A garage is not a dwelling unit, so an ADU that shares a wall only with a garage is not an attached ADU in the Energy Commission's sense. It is classified by the building: an ADU added to an existing garage is an addition, and a garage and ADU built together are new construction.
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Two labels for one wall
HCD's planning definition of an attached ADU is broad: a new dwelling structure that structurally abuts and connects to a primary dwelling or accessory structure, limited to 50 percent of the main house's floor area. The Energy Commission's is narrower: a unit that shares a common wall, floor or ceiling with another dwelling unit. Keep the planning label off the energy report.
Classify by the building
- ADU built onto an existing garage: an addition. The compliance manual treats an ADU created by adding off existing unconditioned space as an addition, and its Figure 9-6 shows one attached to a detached unconditioned structure. No solar is required.
- New garage and ADU built together as one building: newly constructed, because nothing in it has been used or occupied.
- The garage itself turned into the ADU: also an addition, covered in the garage conversion checklist.
If the garage was built without a permit, the building department decides how it is judged.
The wall they share
The garage side is unconditioned, so the shared wall is an envelope wall. Section 150.0(c) sets U-0.095 in 2x4 framing or U-0.069 in 2x6, which R-15 and R-21 meet at 16 inches on center. Where QII applies, the ECC-Rater checks that wall cavities are sealed to the garage as well as to the outdoors and the attic.
Roof and lights in the new-building case
Solar access roof area includes roof space on all other newly constructed structures on the site, so the new garage roof counts when PV is sized, although the garage adds nothing to conditioned floor area in the equation. A new garage also meets Section 150.0(k): at least one garage luminaire is on an occupancy or vacancy sensor.
For a unit stacked over the garage, see ADU above a garage and its floor insulation. The wider split is on addition versus new construction, with the attached ADU and detached ADU reports. Start at ADU Title 24 reports.
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
It depends on the building. If the ADU is built onto an existing garage it is an addition. If the garage and ADU are new together and the unit shares no wall with a dwelling, it is newly constructed.
Yes. Section 150.0(c) applies to above-grade walls separating conditioned from unconditioned spaces, and a garage is unconditioned.
Explore ADU and JADU
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View all ADU and JADU guides → · Permits and Compliance · Title 24 guide