Title 24 Climate Zone 1: ADU Requirements
What the 2025 California Energy Code's prescriptive package requires for a new detached ADU in Climate Zone 1, and how those requirements change the MEP design.
Where Climate Zone 1 is
Of the California places we cover, 12 are mostly in Climate Zone 1, across 4 counties: Del Norte County, Humboldt County, Mendocino County, Sonoma County. Together they issued 441 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 1, the 2025 prescriptive package sets these zone-specific targets.
| Windows | U-factor 0.27 or lower; SHGC No limit; total area up to 20% of floor area |
|---|---|
| Attic, high-performance option | R-38 ceiling; below roof deck: Not required |
| Attic, ducts-inside option | R-38 ceiling; 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 0.793) / 1000 + 1.27; none required under 1.8 kWdc (about 668 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 1
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 65°F in Ferndale to 68°F in Eureka, and the coldest winter design temperature is 28°F in Arcata. 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 1
| Place | County | Electric utility | ADU permits 2021–25 |
|---|---|---|---|
| Eureka | Humboldt | Pacific Gas and Electric (PG&E) | 166 |
| Arcata | Humboldt | Pacific Gas and Electric (PG&E) | 74 |
| Fort Bragg | Mendocino | Pacific Gas and Electric (PG&E) | 41 |
| McKinleyville (unincorp.) | Humboldt | Pacific Gas and Electric (PG&E) | 40 |
| Crescent City | Del Norte | Pacific Power (PacifiCorp) | 35 |
| Mendocino (unincorp.) | Mendocino | Pacific Gas and Electric (PG&E) | 23 |
| Fortuna | Humboldt | Pacific Gas and Electric (PG&E) | 17 |
| Smith River (unincorp.) | Del Norte | Pacific Power (PacifiCorp) | 16 |
| Ferndale | Humboldt | Pacific Gas and Electric (PG&E) | 13 |
| Blue Lake | Humboldt | Pacific Gas and Electric (PG&E) | 6 |
| Rio Dell | Humboldt | Pacific Gas and Electric (PG&E) | 5 |
| Occidental (unincorp.) | Sonoma | 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.
Need MEP plans and a Title 24 report in Climate Zone 1?
We prepare both from the same design, so the energy model and the equipment on the plans agree.
Start My Order