Does unpermitted existing work have to meet Title 24 when I permit an ADU?

Generally yes. The CEC's 2025 ADU FAQ says that when a building was never permitted and you now apply for an ADU permit, the ADU must meet the applicable Energy Code requirements under Section 100.0. Your building department decides how compliance is shown and whether the ADU is treated as a newly constructed building or as an addition.

Why unpermitted work isn't grandfathered

Existing buildings get lighter treatment because they were approved under the rules in effect when they were built. Work that was never permitted has no approved baseline, so the CEC says it must meet the applicable requirements when it enters the permit system. For an ADU application submitted on or after January 1, 2026, that means the 2025 Energy Code.

Common situations

• A garage converted to living space without permits: this would normally be newly conditioned space, which the code treats as an addition, but the building department makes the call. Insulation, windows, HVAC, water heating, lighting and ventilation are then reviewed against the rules for that category.
• A backyard structure built without permits: depending on its history and condition, it may be treated as new construction. That can bring in the PV calculation, battery-ready provisions and other new-construction measures.
• Unpermitted equipment in a permitted building: for example, a mini-split or water heater installed without a permit. Expect it to be documented and verified like new equipment.

What compliance can involve

• Documenting what's actually there: wall, ceiling and floor insulation, window types, and equipment model numbers and efficiency ratings.
• Upgrading components that fall short, or using the performance path to trade off weaker existing elements against stronger new ones.
• ECC-Rater field verification of the measures the compliance documents require, such as refrigerant charge, ventilation airflow and the range hood.
• Possibly opening up finished surfaces if the building department needs to see what's inside walls or ceilings.

How this relates to ADU legalization laws

Separately, California law limits when a city can deny a permit to legalize certain unpermitted ADUs built before January 1, 2020, unless the unit poses a health or safety problem. Those rules govern whether the unit can be legalized and what corrections a city can insist on. How energy compliance is handled for a unit that qualifies is up to your building department, so ask before you start design.

Tips before you apply

1. Gather any records you have: past permits, purchase records and equipment manuals.
2. Photograph walls, attic and equipment labels before any finishes come off.
3. Ask plan check early whether they'll treat the project as an addition or as new construction.

What this means for your MEP plans and Title 24 report

We document the existing conditions you can confirm and the upgrades needed, then model the project under the classification your building department sets. The MEP plans show existing and proposed systems so the inspector can tell what's changing. See the ADU amnesty law and MEP plans, the ADU Title 24 guide, and the garage conversion guide.

Sources: energy.ca.gov, energy.ca.gov, energy.ca.gov.