ADU Solar Requirements in California (2026)
Does your new California ADU need solar? The answer depends on ADU type, size, and whether it's detached. Here's the current Title 24 rule in plain English.
The Short Answer
Detached ADUs built as new construction generally require solar under California Title 24 Part 6 (Building Energy Efficiency Standards). Attached ADUs and ADU conversions of existing space have different requirements and often don't trigger the solar mandate.
When Solar IS Required
- Detached newly constructed ADU of any size in PG&E, SCE, or SDG&E territory.
- Major renovations classified as new construction (typically 75%+ substantial alteration).
- The minimum system size is calculated by Title 24's formula based on conditioned floor area.
When Solar is NOT Required
- Attached ADUs built within or onto an existing home (the existing home's solar system, if any, is deemed sufficient).
- ADU conversions of existing space (garage conversion, basement conversion, interior conversion).
- ADUs with severely shaded roof footprints (Title 24 has a shading exemption — the installer must document it).
- ADUs in municipal utility territories where local building code differs from state Title 24 (rare).
System Size Requirement
Title 24 uses a formula based on conditioned floor area. Typical detached ADU results:
| ADU size | Typical Title 24 PV requirement |
|---|---|
| 400 sq ft junior ADU | 1.0–1.5 kW |
| 600 sq ft 1-bedroom | 1.5–2.0 kW |
| 800 sq ft 1-bedroom | 2.0–2.5 kW |
| 1,000+ sq ft 2-bedroom | 2.5–3.5 kW |
Metering: Attach to Main Home or Separate?
Two options:
- Separate meter. ADU has its own utility account and NEM 3.0 interconnection. Cleaner for rental situations where tenant pays utilities. Higher setup cost.
- Behind-the-meter (shared). ADU solar is part of the main home's meter. Simpler, no new utility account. Works when landlord covers utilities.
Cost Impact on ADU Budget
A 2 kW ADU solar system typically adds $8,000–$12,000 to construction cost. This is usually rolled into the construction loan or home-equity line financing the ADU build. The 30% federal tax credit applies, netting to $5,600–$8,400 after tax credit.
Practical Tips
- Have your ADU designer coordinate with the solar installer during permit drawings — not after.
- Size the main home's solar conservatively if you plan to add an ADU later; it can satisfy the ADU requirement.
- If your ADU site has a significantly shaded roof, pursue the Title 24 shading exemption during permit review.
- Under NEM 3.0, ADU solar pays off much faster with a battery — self-consumption matters.