ADU Skylight Requirements Under Title 24
A skylight in an ADU is fenestration: it takes the window U-factor and SHGC limits, and its area counts toward the glass cap. Each new dwelling unit may have up to 16 square feet at U-0.40 and SHGC 0.30 in climate zones 2, 4 and 6 to 15. Zones 1, 3, 5 and 16 have no SHGC limit.
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Size the allowance against the ADU
The 16 square feet is per dwelling unit, so a detached ADU has its own allowance apart from the main house. It is measured at the rough opening. On a 450 square foot ADU, 16 square feet is about 3.6 percent of the floor, and it still counts inside the 20 percent glass cap of 90 square feet.
Roof pitch changes the count
Section 100.1 defines a skylight as fenestration on a roof less than 60 degrees from horizontal. On a flat or near-flat ADU roof, anything pitched below 1:12 also counts as west-facing whichever way it faces, and in zones 2, 4 and 6 to 15 that limit is 5 percent of floor area. A single 16 square foot skylight would use up about 70 percent of the 22.5 square feet that a 450 square foot unit is allowed.
Unrated skylights lose the allowance
The 0.40 figure needs an NFRC-rated product. Default double-pane skylight U-factors in Table 110.6-A run from 0.84 in a nonmetal frame to 1.30 in metal, and the default SHGC of a fixed double-pane unit is never below 0.55, so a product with no rating cannot use the allowance. The mandatory 0.40 weighted-average ceiling in Section 150.0(q) also includes skylights.
Route by ADU type
- Attached ADU or garage conversion (addition): skylights sit inside the addition glass cap, which at 400 square feet or less is the greater of 75 square feet or 30 percent of the floor area.
- Conversion of an already conditioned room (alteration): an added skylight of up to 16 square feet is exempt from the area limits, though U-factor and SHGC still apply, and a replacement skylight needs U-0.40 and SHGC 0.30.
The general skylight rules are on the residential skylight page. Glass limits for the rest of the walls are on the ADU U-factor and ADU SHGC pages, the unit types are compared on detached ADUs, and reports start on the ADU page.
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
Yes, as fenestration. Up to 3 square feet of tubular skylight with dual-pane diffusers per dwelling unit is excused from the table U-factor and SHGC, but the manual keeps its U-factor at 0.40 or lower.
Yes. Total fenestration under Section 150.1(c)3B includes skylights, so a skylight can use up part of the 20 percent allowance for a new ADU.
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View all ADU and JADU guides → · Permits and Compliance · Title 24 guide