Title 24 Climate Zone 15: ADU Requirements

Where Climate Zone 15 is

Of the California places we cover, 19 are mostly in Climate Zone 15, across 2 counties: Imperial County, Riverside County. Together they issued 2,106 ADU building permits from 2021 through 2025. Zones follow ZIP-code boundaries, so we confirm the zone from the project address before running the Title 24 report.

2025 prescriptive requirements for a new detached ADU

For a new detached ADU in Climate Zone 15, the 2025 prescriptive package sets these zone-specific targets.

Windows	U-factor 0.30 or lower; SHGC 0.20 or lower; total area up to 20% of floor area (west-facing up to 5%)
Attic, high-performance option	R-38 ceiling; below roof deck: R-19
Attic, ducts-inside option	R-38 ceiling plus radiant barrier; ducts and air handler in conditioned space
Framed walls	Assembly U-factor 0.048 or lower
Cool roof	Required: steep-sloped roofs need aged solar reflectance 0.2 or higher and thermal emittance 0.75 or higher; low-sloped roofs 0.63 / 0.75.
Whole-house fan	Not required
Space heating and cooling	Heat pump (other systems through the performance approach)
Water heating	Heat pump water heater; units of 500 sq ft or less, or one bedroom or fewer, have simpler options
Solar PV	kW = (floor area x 1.56) / 1000 + 1.47; none required under 1.8 kWdc (about 212 sq ft or smaller for a single ADU)

An attached ADU or a garage conversion is an addition under the Energy Code, not new construction, so it follows the addition requirements instead. Solar PV is not required for additions.

Design temperatures across Climate Zone 15

The zone sets the energy targets, but the HVAC is sized from the local design temperature, and that varies inside a zone. Among the places we cover here, the 1% summer design temperature ranges from 107°F in Imperial to 112°F in Palm Springs, and the coldest winter design temperature is 24°F in Indio. That spread is why we run a Manual J for each address instead of using a zone-wide rule of thumb.

Places in Climate Zone 15

Place	County	Electric utility	ADU permits 2021–25
Palm Springs	Riverside	Southern California Edison (SCE)	376
Coachella	Riverside	Imperial Irrigation District (IID)	372
El Centro	Imperial	Imperial Irrigation District (IID)	338
Calexico	Imperial	Imperial Irrigation District (IID)	265
Indio	Riverside	Imperial Irrigation District (IID)	175
Cathedral City	Riverside	Southern California Edison (SCE)	122
Imperial	Imperial	Imperial Irrigation District (IID)	110
Palm Desert	Riverside	Southern California Edison (SCE)	98
Desert Hot Springs	Riverside	Southern California Edison (SCE)	59
Brawley	Imperial	Imperial Irrigation District (IID)	43
La Quinta	Riverside	Imperial Irrigation District (IID)	32
Rancho Mirage	Riverside	Southern California Edison (SCE)	25
Heber (unincorp.)	Imperial	Imperial Irrigation District (IID)	25
Indian Wells	Riverside	Southern California Edison (SCE)	22
Holtville	Imperial	Imperial Irrigation District (IID)	14
Bermuda Dunes (unincorp.)	Riverside	Imperial Irrigation District (IID)	10
Thousand Palms (unincorp.)	Riverside	Imperial Irrigation District (IID)	8
Westmorland	Imperial	Imperial Irrigation District (IID)	7
Mecca (unincorp.)	Riverside	Imperial Irrigation District (IID)	5

Sources

• 2025 Building Energy Efficiency Standards (Title 24, Part 6), Section 150.1(c) and Table 150.1-A
• 2025 Single-Family Residential Compliance Manual (CEC-400-2025-005)

Requirements shown are the 2025 prescriptive package (effective January 1, 2026) for newly constructed single-family buildings, which includes new detached ADUs. A performance (computer-modeled) approach can trade some items against others. Your Title 24 report confirms what applies to your project.