Attached ADU Title 24 Checklist
An attached ADU shares a wall, floor or roof with the house, so its checklist is about the join: choose between modeling the addition alone or with the house, decide whether to extend ducts or install a separate system, feed the load calculation, and document any reused windows. Solar is not on the list.
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1. Choose the compliance route
On the performance path an addition can be modeled alone, or together with the existing house and any alterations. Combined modeling lets a strong addition offset a weak existing feature. If the existing walls have at least R-11 in the framing and the combined model complies, they need not also meet the mandatory wall insulation of Section 150.0(c).
2. Decide how the ADU is heated
- Separate system: new equipment meets the new-home rules for the unit.
- Extended ducts: the combined existing and new system must test at 10 percent total leakage or 7 percent to outside. If neither is possible, an ECC-Rater smoke-tests and verifies that every accessible leak is sealed.
- Existing ducts built, insulated or sealed with asbestos are excepted from the sealing rule.
3. Feed the load calculation
Size the equipment from a calculation that uses the shared-system block load if one system serves several rooms, and that assumes no more envelope leakage than Table 150.2-C allows unless a blower-door test says otherwise. Without verified airflow of 350 cfm per ton, equipment capacity is capped; ductless systems are exempt.
4. List reused windows
A window taken out of the house and reused in the ADU counts as a new window. A prescriptive design must use its actual or default U-factor and SHGC, so a tired old unit may force the performance path.
5. What drops away
Solar PV is not required, and the detached unit's flat 20 percent glazing cap gives way to the addition limits of Section 150.2(a)1, which allow a larger share of glass on additions of 700 square feet or less. Whole-dwelling ventilation, a thermostat of its own and no shared return air still apply.
The rule for each item is on the attached ADU report page. See also HVAC load calculations, duct leakage testing, alteration versus addition and 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
Yes. Extending any length of duct brings the existing and new ducts under the sealing and testing rules, unless the existing system contains asbestos.
Yes, but it is treated as a new window, so its actual or default U-factor and SHGC must be used to show compliance.
Explore ADU and JADU
- California ADU Rules and Title 24
- Can a New ADU Use a Gas Furnace?
- Can a New ADU Use a Gas Water Heater?
- Can an ADU Use a Mini-Split Heat Pump?
- Can Existing Solar Panels Count for a New ADU?
View all ADU and JADU guides → · Permits and Compliance · Title 24 guide