ADU Alteration vs Addition: Which Title 24 Rules Apply?
An ADU is an addition when it increases both conditioned floor area and conditioned volume, as a garage conversion does. It is an alteration when it repurposes space that is already conditioned, such as a basement or pool house. An alteration regulates only the components being changed; an addition regulates the whole new space.
- Choose your report
- Upload your plans
- Pay securely
- Permit-ready report by email
One question sorts the project
Was the space conditioned before? The Energy Commission puts conversions of unconditioned space, attached or detached, under the addition rules of Section 150.2(a). A finished room, a conditioned basement, or a conditioned pool house, studio or guest house becomes an ADU by alteration, under Section 150.2(b), because nothing grows.
What the label changes
- Credit for existing conditions. Walls around newly conditioned space count as new in an addition, so existing conditions cannot be verified for credit there. In an alteration, an ECC-Rater can verify existing insulation or windows, and Table 150.2-G then takes the existing value as the standard design instead of the current requirement.
- Forms. A prescriptive alteration uses CF1R-ALT-01-E in the manual's Appendix A list (its chapter 9 names CF1R-ALT-02-E, so confirm with the building department), or CF1R-ALT-05-E without field verification. An addition uses CF1R-ADD-01-E or CF1R-ADD-02-E.
- Size relief. The 700 and 400 square foot packages exist only for additions.
Alteration triggers to check
- New or replacement ducts over 25 feet bring duct sealing and testing.
- Added glazing up to 16 square feet skips the area caps. Replacing windows of the same total area meets the U-factor and SHGC limits, with a relief for up to 75 square feet at U-0.40.
- Altered lighting meets Section 150.0(k); luminaires you leave alone may stay.
The compliance manual lists adding insulation to an existing ceiling, wall or floor, reroofing and replacing windows as alterations.
When one permit holds both
A garage conversion ADU next to an alteration of the house can be modeled together, with the existing house, the addition and the ADU as separate zones. Any addition or alteration may instead comply as a whole building under Section 150.2(c).
Examples on the site: a basement conversion, the JADU checklist and the garage conversion ADU. Compare new construction on the addition versus new construction page, or see ADU Title 24 reports.
Common Title 24 Questions
- Do ADUs need Title 24 in California?
- What plans are needed for an ADU Title 24 report?
- How much does an ADU Title 24 report cost?
- How long does an ADU Title 24 report take?
- Does a garage conversion ADU need Title 24?
- Does a JADU need a Title 24 report?
Frequently Asked Questions
No. The Energy Commission treats it as an alteration, because it does not increase conditioned floor area or volume. Only altered components that the Energy Code covers are triggered.
A prescriptive alteration uses CF1R-ALT-01-E or, when no field verification is required, CF1R-ALT-05-E; the manual's chapter 9 also names CF1R-ALT-02-E, so confirm with the building department. The performance path produces a CF1R-PRF-01-E.
Explore ADU and JADU
- ADU Attached to a Garage: How Title 24 Classifies the Project
- ADU Attic and Roof Insulation Requirements
- ADU Bathroom Exhaust Fan Requirements
- ADU Battery-Ready Requirements Under the 2025 Energy Code
- What Is the CF1R for an ADU?
View all ADU and JADU guides → · Permits and Compliance · Title 24 guide