Title 24 Climate Zone 10: ADU Requirements

Where Climate Zone 10 is

Of the California places we cover, 49 are mostly in Climate Zone 10, across 3 counties: Riverside County, San Bernardino County, San Diego County. Together they issued 9,172 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 10, the 2025 prescriptive package sets these zone-specific targets.

Windows	U-factor 0.30 or lower; SHGC 0.23 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.
Whole-house fan	Required, 1.5 cfm per sq ft of floor area (units of 500 sq ft or less exempt)
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 0.627) / 1000 + 1.41; none required under 1.8 kWdc (about 622 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 10

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 85°F in Spring Valley to 104°F in San Jacinto, and the coldest winter design temperature is 20°F in Hemet. 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 10

Place	County	Electric utility	ADU permits 2021–25
Riverside	Riverside	Riverside Public Utilities	1,097
San Bernardino	San Bernardino	Southern California Edison (SCE)	854
Fontana	San Bernardino	Southern California Edison (SCE)	754
Ontario	San Bernardino	Southern California Edison (SCE)	563
Escondido	San Diego	San Diego Gas & Electric (SDG&E)	446
Hemet	Riverside	Southern California Edison (SCE)	317
Rancho Cucamonga	San Bernardino	Southern California Edison (SCE)	316
El Cajon	San Diego	San Diego Gas & Electric (SDG&E)	311
Moreno Valley	Riverside	Southern California Edison (SCE)	300
Upland	San Bernardino	Southern California Edison (SCE)	285
Jurupa Valley	Riverside	Southern California Edison (SCE)	284
Chino	San Bernardino	Southern California Edison (SCE)	269
Spring Valley (unincorp.)	San Diego	San Diego Gas & Electric (SDG&E)	262
Corona	Riverside	Southern California Edison (SCE)	256
Yucaipa	San Bernardino	Southern California Edison (SCE)	239
Redlands	San Bernardino	Southern California Edison (SCE)	207
Poway	San Diego	San Diego Gas & Electric (SDG&E)	154
San Marcos	San Diego	San Diego Gas & Electric (SDG&E)	153
Highland	San Bernardino	Southern California Edison (SCE)	152
Fallbrook (unincorp.)	San Diego	San Diego Gas & Electric (SDG&E)	146
Rialto	San Bernardino	Southern California Edison (SCE)	132
Montclair	San Bernardino	Southern California Edison (SCE)	129
Loma Linda	San Bernardino	Southern California Edison (SCE)	125
Lakeside (unincorp.)	San Diego	San Diego Gas & Electric (SDG&E)	120
Ramona (unincorp.)	San Diego	San Diego Gas & Electric (SDG&E)	112
Santee	San Diego	San Diego Gas & Electric (SDG&E)	97
Norco	Riverside	Southern California Edison (SCE)	96
Murrieta	Riverside	Southern California Edison (SCE)	93
Chino Hills	San Bernardino	Southern California Edison (SCE)	91
Perris	Riverside	Southern California Edison (SCE)	81
Menifee	Riverside	Southern California Edison (SCE)	80
Valley Center (unincorp.)	San Diego	San Diego Gas & Electric (SDG&E)	74
Colton	San Bernardino	Colton Electric Utility	70
Temecula	Riverside	Southern California Edison (SCE)	64
Eastvale	Riverside	Southern California Edison (SCE)	61
Alpine (unincorp.)	San Diego	San Diego Gas & Electric (SDG&E)	60
Lake Elsinore	Riverside	Southern California Edison (SCE)	44
San Jacinto	Riverside	Southern California Edison (SCE)	40
Homeland (unincorp.)	Riverside	Southern California Edison (SCE)	35
Wildomar	Riverside	Southern California Edison (SCE)	32
Winchester (unincorp.)	Riverside	Southern California Edison (SCE)	32
Bonsall (unincorp.)	San Diego	San Diego Gas & Electric (SDG&E)	32
Mira Loma (Jurupa Valley)	Riverside	Southern California Edison (SCE)	31
Calimesa	Riverside	Southern California Edison (SCE)	27
Beaumont	Riverside	Southern California Edison (SCE)	24
Jamul (unincorp.)	San Diego	San Diego Gas & Electric (SDG&E)	23
Grand Terrace	San Bernardino	Southern California Edison (SCE)	15
Bloomington (unincorp.)	San Bernardino	Southern California Edison (SCE)	11
Nuevo (unincorp.)	Riverside	Southern California Edison (SCE)	7

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.