Title 24, Part 6 of the California Building Standards Code—known as the California Energy Code—governs the energy efficiency design of all new construction and major renovations in California, including accessory dwelling units. Every ADU building permit application must include a Title 24 energy compliance report demonstrating that the proposed design meets the current standards. This requirement applies regardless of ADU size or type: garage conversion, attached addition, or new detached structure all require a compliant energy report before a building permit can be issued.
The Four Systems Title 24 Regulates
The California Energy Code addresses four primary building systems: the building envelope (walls, roof, windows, doors, and insulation), space heating and cooling equipment, domestic water heating, and lighting. For an ADU, compliance is demonstrated through either the Prescriptive path or the Performance path—two distinct methods with different documentation requirements and trade-off flexibility.
California's 16 Climate Zones
California is divided into 16 climate zones, each with different energy requirements. The climate zone is determined by the project's location. Zone assignments matter significantly: Climate Zone 6 (the San Diego coast) has much lower heating load requirements than Climate Zone 16 (the Sierra Nevada foothills), while Climate Zone 9 (the San Fernando Valley) imposes stricter cooling-related requirements. Your energy consultant will identify the correct climate zone and apply the corresponding mandatory measures.
The Solar PV Mandate
One of the most significant recent additions to the California Energy Code is the mandatory photovoltaic requirement. New ADUs over 500 square feet must include a solar PV system sized to offset the projected annual electricity use of the ADU. This is a firm requirement for new construction—not a credit or incentive. However, the solar system does not need to be installed on the ADU itself. If the primary dwelling already has a compliant solar system, that system can be expanded to meet the ADU's solar requirement.
Prescriptive vs. Performance Compliance
The Prescriptive path requires the design to meet a checklist of minimum performance thresholds for each building component. Insulation must achieve specified R-values (typically R-38 or better at the ceiling, R-15 or better in walls for most zones). Windows must meet maximum U-factor and SHGC ratings. The prescriptive path is straightforward but inflexible.
The Performance path uses energy modeling software—most commonly CBECC-Res—to demonstrate that the total modeled energy use of the proposed design is no greater than a compliant reference design. This allows trade-offs: a high-performance roof can offset windows with a higher U-factor, or an efficient heat pump water heater can compensate for slightly less wall insulation. The performance path requires a certified energy consultant.
Envelope Requirements
For a typical ADU in most California climate zones under the 2022 code, prescriptive requirements include: ceiling insulation of at least R-38, wall cavity insulation of R-15, floor insulation of R-19 over unconditioned space, and windows with U-factor no greater than 0.30 and SHGC no greater than 0.23 in most southern zones. These values vary by climate zone.
Mechanical Systems: Heat Pumps and Water Heaters
Heat pump water heaters have become the standard for domestic hot water in new California ADUs. They are 2–3 times more energy efficient than conventional electric resistance units and are required by the 2022 code in many climate zones for new construction. Heat pump HVAC systems (mini-split systems) are also commonly specified for ADUs because they provide both heating and cooling in a single compact unit, require no ductwork, and meet or exceed Title 24 efficiency minimums.
Many California jurisdictions—including those throughout the San Francisco Bay Area, Los Angeles County, and San Diego—have adopted local reach codes that require all-electric systems in new ADUs, prohibiting gas appliances and gas service altogether. An all-electric ADU design simplifies the compliance picture and eliminates gas service costs entirely.
HERS Verification
Some Title 24 compliance measures require field verification by a certified Home Energy Rating System (HERS) rater before a certificate of occupancy can be issued. Common HERS-verified measures include duct leakage testing, refrigerant charge verification for air conditioning systems, and verification of quality insulation installation. A HERS rater inspection must be scheduled and completed before the final building inspection.
Practical Application
Title 24 compliance is not a barrier to building an ADU in California—it is a design input that shapes your mechanical system selection, insulation specification, and window choices. Working with a design team that handles energy compliance as part of the permit package prevents the most common delays: submitting drawings with windows that don't comply or a mechanical schedule with below-minimum equipment. Getting the compliance report right on the first submission keeps your permit on track.