ADU Bathroom Exhaust Fan Requirements
Each bathroom in a new ADU or addition needs its own mechanical exhaust fan, window or not, at 50 cfm on demand or 20 cfm continuous. In a compact ADU the useful decision is whether one continuous fan also serves as the whole-dwelling ventilation fan, which brings an ECC-Rater airflow test.
- Choose your report
- Upload your plans
- Pay securely
- Permit-ready report by email
The one-fan decision
A compact ADU needs little whole-dwelling air: 27 cfm for a 400 square foot studio, 33 cfm for a 600 square foot one-bedroom. A single continuous bathroom fan rated at or above that figure can do both jobs, and Example 4-6 in the compliance manual uses a bathroom fan this way. Two continuous bathroom fans add together toward the total (Example 4-8). The trade-off is paperwork. A fan that is the ventilation system must be rated 1.0 sone or less and have its airflow measured by the ECC-Rater, while a stand-alone demand fan, allowed up to 3 sones, is checked only by the building inspector.
What CALGreen adds
Section 4.506.1 of CALGreen asks for an ENERGY STAR fan ducted outside with a humidity control adjustable from 50 to 80 percent relative humidity, unless the fan is a component of a whole-house ventilation system. A continuous whole-dwelling fan therefore skips the humidity control. CALGreen counts a bathroom as a room with a tub, shower or tub-shower combination; the Energy Code also counts a powder room.
What the project type changes
- New detached ADU: a fan in each bathroom, with no size exception under Section 150.0(o).
- Addition or JADU addition: local exhaust under Section 150.2(a)1Cii applies even though a JADU addition is excused from whole-dwelling airflow.
- Converted room: only an altered or replacement fan is caught. It meets Section 150.0(o)1G, carries airflow and sound ratings, and is rated at no less than the airflow required.
Installation notes
A fan that exchanges air with the outdoors needs a backdraft or automatic damper. A duct run of 25 feet or less with no more than three elbows lets the installer prove airflow from Table 150.0-H rather than measuring it. The building inspector looks at the housing and duct at framing and at the noise rating at final.
Fan labels and duct sizes are on the bathroom exhaust fan page. See also ADU whole-house ventilation, the JADU checklist, kitchen range hoods and the ADU page.
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. Section 150.0(o)1G requires local mechanical exhaust in each bathroom and lists no window exception.
The compliance manual shows a bathroom fan providing the dwelling ventilation. It must then meet the whole-dwelling airflow, be rated at 1.0 sone or less and pass the ECC-Rater airflow measurement.
Explore ADU and JADU
- ADU Battery-Ready Requirements Under the 2025 Energy Code
- What Is the CF1R for an ADU?
- ADU Cool Roof Requirements by Climate Zone
- ADU Duct Leakage Testing: When Is It Required?
- ADU Electric-Ready Requirements for Gas Appliances
View all ADU and JADU guides → · Permits and Compliance · Title 24 guide