The general rule
California's Title 24 solar mandate was extended to cover Accessory Dwelling Units starting in 2023. In general terms: if you're building a brand-new, site-built, detached ADU, solar panels are generally required as a condition of your building permit. If you submit your ADU permit in 2026, you fall under the 2025 Title 24 standards, which took effect January 1, 2026; projects permitted before that date generally follow the prior 2022 code. Some cities spell this out in their own ADU standards — Santa Clarita, for instance, explicitly requires solar on new detached ADUs (see our Santa Clarita ADU guide).
How the required system size is determined
Title 24 uses a specific sizing formula (Equation 150.1-C) that factors in your ADU's conditioned floor area and your California climate zone. As a general guide, ADUs in the roughly 600–1,200 square foot range often require somewhere in the neighborhood of 1.8–3.5 kW of solar capacity — commonly around 4 to 8 panels, depending on panel wattage. Your actual required system size depends on your specific project and should be calculated by your solar installer or energy consultant, not estimated from this general range.
Real exemptions worth knowing about
The newer battery-ready wiring requirement
Battery storage itself is not generally mandated for most single-family ADUs. However, the 2025 Title 24 code (effective for 2026 permits) generally requires "battery-ready" pre-wiring — meaning your ADU must include a dedicated 240V circuit and reserved panel space for a future battery system, even if you don't install the battery itself right away.
This is general information, not an energy code determination
Title 24 requirements involve specific calculations tied to your exact project and climate zone. This page describes the general framework, not a determination for your specific ADU. A licensed solar installer, energy consultant, or your local building department can confirm exact requirements and available exemptions for your project.
Frequently asked questions
Generally, yes for newly built, site-built detached ADUs — but real exemptions exist for heavily shaded roofs, cases where the required system size is very small, and factory-built units. Confirm your specific situation with a licensed solar installer or your building department.
Possibly — if your roof has fewer than 80 contiguous square feet of viable solar area due to permanent, fixed shading, the requirement can generally be waived, provided it's documented in your permit application.
Generally no, not yet, for most single-family ADUs — but the current code generally requires reserved wiring and panel space so a battery can be added later without major rework.