ADU Addition vs New Construction: Which Title 24 Rules Apply?
An ADU is new construction when it is a new building that has never been used or occupied and shares no wall, floor or ceiling with a dwelling. It is an addition when it adds conditioned floor area and volume to an existing building. The label decides solar, forms, testing limits and size relief.
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Two tests, in order
First, is the building new? The Energy Code calls a building newly constructed if it has never been used or occupied for any purpose, and the compliance manual places a new detached structure with no common walls there. If not, ask whether the work increases both conditioned floor area and conditioned volume of an existing building. Both must grow, which is why the manual lists a floor-to-ceiling bay window and an enclosed patio as additions.
Borderline cases the sources settle
- A breezeway joining a new ADU to the house leaves it new construction.
- A second story over an existing detached garage is an addition, because the ADU is built off existing unconditioned space.
- Legalizing conditioned space added earlier without a permit is an addition in the manual, but the Commission leaves the building department to decide whether it is judged as an addition or as a new building.
What the label changes
- Solar: a new detached ADU needs newly installed PV unless an exception applies. An addition does not.
- Forms: CF1R-NCB-01-E covers new buildings and prescriptive additions of 1,000 square feet or more; CF1R-ADD-01-E covers smaller additions.
- Load calculation: for an addition, assumed envelope leakage may not exceed Table 150.2-C, and equipment capacity is capped unless airflow is verified at 350 cfm per ton.
- Relief: the 700 and 400 square foot packages apply only to additions.
When one permit holds both
The floor areas of several separate additions under one permit are added together. A house, an addition and a converted-garage ADU can be modeled as separate zones. Where changes to the house are extensive, the manual allows existing plus addition to be modeled as a newly constructed building, which brings every existing feature up to current code. Any addition may also comply as a whole building under Section 150.2(c).
Next, read the detached or attached ADU page, the alteration comparison, the first ADU guide and HVAC load calculations. Our ADU Title 24 reports use the label you confirm with the building department.
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 shows a newly constructed ADU with an optional breezeway as new construction. It is attached only when it shares a wall, floor or ceiling with another dwelling unit.
The building department. The Energy Commission says the unit must meet the Energy Code that applies now, and the department determines whether it complies as a newly constructed building or as an addition.
Explore ADU and JADU
- ADU Airflow Verification: When Is It Required?
- ADU Alteration vs Addition: Which Title 24 Rules Apply?
- ADU Attached to a Garage: How Title 24 Classifies the Project
- ADU Attic and Roof Insulation Requirements
- ADU Bathroom Exhaust Fan Requirements
View all ADU and JADU guides → · Permits and Compliance · Title 24 guide