Title 24 Climate Zone 4: ADU Requirements

Where Climate Zone 4 is

Of the California places we cover, 26 are mostly in Climate Zone 4, across 5 counties: Monterey County, San Benito County, San Luis Obispo County, San Mateo County, Santa Clara County. Together they issued 8,229 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 4, the 2025 prescriptive package sets these zone-specific targets.

Windows	U-factor 0.27 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-30 ceiling plus radiant barrier; ducts and air handler in conditioned space
Framed walls	Assembly U-factor 0.048 or lower
Cool roof	Not required
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 0.586) / 1000 + 1.21; none required under 1.8 kWdc (about 1,007 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 4

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 83°F in Palo Alto to 94°F in Paso Robles, and the coldest winter design temperature is 16°F in Paso Robles. 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 4

Place	County	Electric utility	ADU permits 2021–25
San Jose	Santa Clara	Pacific Gas and Electric (PG&E)	2,399
Campbell	Santa Clara	Pacific Gas and Electric (PG&E)	1,097
Palo Alto	Santa Clara	City of Palo Alto Utilities	723
Los Gatos	Santa Clara	Pacific Gas and Electric (PG&E)	450
Saratoga	Santa Clara	Pacific Gas and Electric (PG&E)	387
Los Altos	Santa Clara	Pacific Gas and Electric (PG&E)	384
Santa Clara	Santa Clara	Silicon Valley Power	378
Sunnyvale	Santa Clara	Pacific Gas and Electric (PG&E)	377
Mountain View	Santa Clara	Pacific Gas and Electric (PG&E)	317
Los Altos Hills	Santa Clara	Pacific Gas and Electric (PG&E)	277
Milpitas	Santa Clara	Pacific Gas and Electric (PG&E)	229
Cupertino	Santa Clara	Pacific Gas and Electric (PG&E)	191
East Palo Alto	San Mateo	Pacific Gas and Electric (PG&E)	164
Gilroy	Santa Clara	Pacific Gas and Electric (PG&E)	162
Paso Robles	San Luis Obispo	Pacific Gas and Electric (PG&E)	108
Atascadero	San Luis Obispo	Pacific Gas and Electric (PG&E)	107
Morgan Hill	Santa Clara	Pacific Gas and Electric (PG&E)	103
Greenfield	Monterey	Pacific Gas and Electric (PG&E)	88
Hollister	San Benito	Pacific Gas and Electric (PG&E)	74
Monte Sereno	Santa Clara	Pacific Gas and Electric (PG&E)	67
King City	Monterey	Pacific Gas and Electric (PG&E)	65
Templeton (unincorp.)	San Luis Obispo	Pacific Gas and Electric (PG&E)	33
San Martin (unincorp.)	Santa Clara	Pacific Gas and Electric (PG&E)	18
San Miguel (unincorp.)	San Luis Obispo	Pacific Gas and Electric (PG&E)	16
San Juan Bautista	San Benito	Pacific Gas and Electric (PG&E)	10
Creston (unincorp.)	San Luis Obispo	Pacific Gas and Electric (PG&E)	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.