Does my ADU need a whole-house fan, and what is the 500 sq ft exemption?

Under the prescriptive path of the 2025 Energy Code, a new ADU in climate zones 8 through 14 needs a whole-house fan unless it has 500 square feet or less of conditioned floor area. ADUs built as additions of 1,000 square feet or less, which covers most garage conversions, are also exempt, and the performance path lets you model the ADU without one.

Where the requirement comes from

Section 150.1(c)12 (ventilation cooling) points to Table 150.1-A, which marks a whole-house fan as required in climate zones 8, 9, 10, 11, 12, 13 and 14 and not required in zones 1 through 7, 15 and 16. Reference Appendix JA2 names the reference cities for those zones: Fullerton (8), Burbank-Glendale (9), Riverside (10), Red Bluff (11), Sacramento (12), Fresno (13) and Palmdale (14). Your zone depends on your parcel, not the reference city.

The two exemptions

• 500 square feet or less: the exception to Section 150.1(c)12 says new dwelling units with 500 square feet or less of conditioned floor area do not have to meet the whole-house fan requirement.
• Additions of 1,000 square feet or less: Exception 5 to Section 150.2(a) exempts these from ventilation cooling. The CEC's ADU FAQ treats attached ADUs and conversions of unconditioned space, such as garages, as additions.

If a fan is required

• Total airflow of at least 1.5 cfm per square foot of conditioned floor area, using the airflow listed in the Home Ventilating Institute Certified Products Directory. A 600 square foot ADU, for example, needs at least 900 cfm.
• At least 1 square foot of attic vent free area per 750 cfm of fan airflow, or more if the manufacturer specifies it. That 900 cfm fan needs at least 1.2 square feet. Fans vented directly to the outside are exempt from this attic vent rule.
• A one-page "How to operate your whole-house fan" sheet for the occupant.

Using the performance path instead

The CEC's Compliance Manual notes that a whole-house fan is not a mandatory measure; it is required only for prescriptive compliance. On the performance path, the software can model no fan, a default fan or an ECC-verified fan. Leaving it out in a zone where the standard design assumes one means the energy budget must be met with other features. That can be the practical route for an ADU with vaulted ceilings and no attic.

What it means for your mechanical plan

• Confirm your climate zone and conditioned floor area early, since together they decide whether a fan is required at all.
• If a fan is required, show its location, model, rated cfm and the attic vent free area calculation.
• If you use the performance path, make sure the plans match what the Title 24 report models.

See prescriptive vs. performance compliance and your ADU's climate zone.

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