Two-Story ADU Title 24 Requirements
A two-story ADU is classified like any other: a new detached building is new construction, while a second story on the house or over an existing detached garage is an addition. The extra floor does not change the category, but it doubles conditioned floor area, which drives solar size, the glazing cap and the ventilation rate.
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Which category a second story falls in
A stand-alone two-story ADU is a newly constructed building. HCD lists an ADU built as a new second story of a single-family dwelling as an attached ADU, and the compliance manual calls a second story over an existing detached garage an addition. State ADU law sets no story limit, and a local agency must allow two where height limits and building standards permit.
Every floor counts
Conditioned floor area is measured on all floors, at the floor level of the exterior wall surfaces, so two 500 square foot floors make 1,000. That matters three ways.
- Solar. Equation 150.1-C is kWdc = (CFA x A)/1000 + (dwelling units x B). In climate zone 12, A is 0.613 and B is 1.40. A one-story 500 square foot unit works out to 1.71 kW, below the 1.8 kWdc exception, so no PV is required. The two-story 1,000 square foot unit works out to 2.01 kW, so PV is required unless another exception applies.
- Glazing. The new-building package caps total glass at 20 percent of floor area: 200 square feet on the two-story unit, 100 on the single floor.
- Ventilation. Required airflow is 0.03 cfm per square foot of floor area plus 7.5 cfm for each bedroom plus one extra. For a detached unit, the infiltration credit rises with the height between its lowest and highest above-grade points, measured against 8.2 feet. A unit stacked above or below another dwelling unit may not take that credit.
Between the floors
The floor between two conditioned stories is not a raised floor under Section 150.0(d), which covers floors over unconditioned space or outside air. Only a floor over a garage or open air carries the insulation rule, covered under floor insulation over a garage.
A second story on the house is an addition with no PV requirement, set out on the attached ADU and second-story addition pages. A new two-story unit follows the detached ADU page, and height limits are on California ADU rules. 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
No. It is an attached ADU, which the Energy Commission treats as an addition, and additions are exempt from the photovoltaic requirement.
Yes. The 20 percent cap in the new-building package is a share of conditioned floor area on all floors, so a second floor raises the allowed glass area.
Explore ADU and JADU
- Title 24 for ADU Builders
- Title 24 for an ADU Above a Garage
- Attached ADU Title 24 Report
- Detached ADU Title 24 Report
- ADU Addition vs New Construction: Which Title 24 Rules Apply?
View all ADU and JADU guides → · Permits and Compliance · Title 24 guide