Title 24 for ADUs: What California Homeowners Need to Know

If your ADU is site-built or factory-built and needs a building permit, it needs to show compliance with California's energy code (Title 24, Part 6). HUD-code manufactured homes follow a different set of state rules. In practice that means a CF1R compliance report submitted with your plans, installer certificates (CF2R) from the trades, and field verification by a certified HERS or ECC rater (CF3R) for any measures the CF1R flags for testing. When field verification is required, the building department will not sign off final inspection until that paperwork is complete and registered.

The energy code does not have a separate ADU chapter. Instead, your ADU is treated like whatever it physically is. A newly constructed detached ADU is treated as new single-family construction. An attached ADU that adds new floor area is generally treated as an addition to the existing house. A garage conversion turns unconditioned space into conditioned space, which is also generally handled as an addition. Converting space that is already conditioned, such as a bonus room or part of the main house, is usually handled as an alteration.

That classification drives almost everything else: which requirements apply, whether solar is required, and how much testing a HERS or ECC rater has to do. Confirm the classification with your energy consultant and building department before you finalize plans.

Under the solar PV requirement in Section 150.1(c)14, newly constructed detached ADUs generally must include a solar PV system sized by the code formula. Attached ADUs, garage conversions and conversions of existing space generally do not trigger the new-construction PV requirement because they are additions or alterations, not new buildings.

There are exceptions even for detached ADUs. The most common are limited solar access (less than 80 contiguous square feet of usable roof), and a calculated minimum system size below 1.8 kWdc, which small ADUs in mild climate zones sometimes hit. Your CF1R will show whether PV is required and at what size.

Your CF1R has the final list. For a 2025-code ADU it commonly includes duct leakage testing if you have ducted heating or cooling, airflow and fan efficacy checks on ducted systems, refrigerant charge verification on heat pumps (and on air conditioners in Climate Zones 2 and 8 through 15), and verification of measures like Quality Insulation Installation (QII). A new ADU dwelling unit also needs its whole-house ventilation airflow and its kitchen range hood verified, even if it has no ducts. A ductless mini-split has no ducts to leakage-test, so it usually means fewer field tests than a ducted system, but it still needs refrigerant charge verification and anything else the CF1R lists.

QII is the one that catches people out. Under the 2025 prescriptive rules it applies to new detached ADUs in every climate zone and to additions over 700 square feet. Additions of 700 square feet or less are exempt unless the energy model takes credit for QII. Either way, the rater has to inspect at framing and again before drywall goes up. Miss that window and you may have to open walls or redo the energy calculations.

For a new detached ADU, the 2025 code's baseline uses a heat pump for both space heating and water heating in every climate zone. Under the 2022 code (permit applications before January 1, 2026), the baseline used a heat pump for space heating or water heating depending on climate zone. You can still propose gas equipment on the performance path, but the energy model has to show the project still complies, usually by making up the difference elsewhere. Attached ADUs and conversions follow the addition and alteration rules instead, including separate rules for equipment you keep. For a new detached ADU, an all-electric design (a ductless or small ducted heat pump plus a heat pump water heater) lines up with the 2025 baseline, so it is worth pricing first.

Get the CF1R done before plan submittal, not after the first plan check comment. Book your HERS or ECC rater early and tell them whether QII is on the report. Give your HVAC installer the list of tests on the CF1R, such as refrigerant charge and, for ducted systems, duct leakage, and have them complete the CF2R promptly. Leaky ducts and missing paperwork are common reasons a verification gets held up, and both are easier to fix before the rater arrives than after.