Title 24 for an ADU Above a Garage
It depends on whether the garage already exists: an ADU built on top of an existing garage is an addition, while a new building with the garage below and the ADU above is newly constructed. Either way, the floor over the garage is a raised floor that needs R-19 insulation in wood framing, or an assembly U-factor no higher than 0.037.
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Three projects that look alike from the street
Over an existing detached garage. The single-family compliance manual names this case: an ADU created by adding off an existing unconditioned space, such as building a second story over an existing detached garage building, is an addition. The Energy Commission’s ADU FAQs likewise list new conditioned space added to a detached garage among their addition examples. Solar PV is not required, and the relief for additions of 700 square feet or less is available.
Over an existing attached garage. HCD’s handbook lists an ADU built on top of an attached garage as an attached ADU. In energy terms it adds conditioned floor area and volume to the house, so it is an addition too, and it brings the shared-building rules with it, above all that it cannot take return air from the house’s ducted system. Those are on the attached ADU page.
A new garage and ADU built together. When both floors are new and the building stands apart from the house, nothing in it has been used or occupied, so it is a newly constructed building. The ADU meets the new-home package, solar PV included unless an exception applies, as set out for a detached ADU.
Only the conditioned floor counts
The Energy Code defines a garage as a nonhabitable space where vehicles are parked, and it stays unconditioned under the ADU. Conditioned floor area counts only enclosed conditioned space, so the garage adds nothing to the numbers that drive the rules: the 700 and 400 square foot thresholds for an addition, or the floor area in the PV sizing equation for a new building.
That changes when part of the garage is converted at the same time. The manual’s Example 9-8 is a 400 square foot garage conversion with a 400 square foot room built above it. The two count together as an 800 square foot addition, so QII applies prescriptively. Garage walls that keep their existing cladding are spared QII header insulation and air sealing, and take R-15 in 2x4 or R-21 in 2x6 framing. Converting the whole garage is covered under garage conversion ADUs.
The floor over the garage
Because it sits over unconditioned space, the floor between garage and ADU is a raised floor in Energy Code terms, and a demising partition that forms part of the building envelope. Section 150.0(d) makes its insulation mandatory: an assembly U-factor no greater than 0.037, which a wood-framed floor meets with R-19. Being mandatory, it holds in every climate zone, for additions and new buildings alike, and on either path: the manual notes that a performance model may assume less floor insulation, but R-19 still has to be installed.
Where the garage is bigger than the unit and shares an open attic with it, the manual’s Example 3-18 says the whole roof or ceiling is insulated, including the part over the garage, unless a continuous air barrier separates the two attic spaces.
Between the garage and the unit
- A swinging door between garage and ADU that must carry a fire rating is exempt from the U-0.20 door value, under the exception to Section 150.1(c)5.
- Walls and openings between the dwelling and an attached garage are sealed or gasketed so that air does not pass between them, and ventilation air may not be drawn from the garage.
- A heat pump water heater can sit in the garage. The new-home package allows the tank in the garage or in conditioned space; the addition rules bar only an outdoor tank and call for a rigid R-10 base.
What the report needs from you
Beyond the usual ADU drawings: a section through the garage ceiling showing the floor framing depth and insulation, whether the garage is existing or new and which of its walls stay, and where the water heater and any air handler will go. If the garage or any part of the structure was built without a permit, the Commission’s ADU guidance says the unit must meet the current code and leaves the building department to decide whether it complies as a newly constructed building or as an addition. That is worth settling before the report is ordered; unpermitted work explains the wider picture.
Other ADU Types
Frequently Asked Questions
No. The single-family compliance manual treats a second story over an existing detached garage as an addition, so solar PV is not required and Section 150.2(a) applies.
Not while it stays unconditioned. The Energy Code works from conditioned floor area, so the garage below is left out; any part of it converted under the same permit is added to the total.
At least R-19 in a wood-framed floor, or an assembly U-factor of 0.037 or lower. It is a mandatory measure under Section 150.0(d), so it applies in every climate zone and on either compliance path.
Yes, unless an exception applies, because the building is newly constructed. The PV size is calculated from the conditioned floor area and the number of dwelling units, so the garage does not add to it.