Is a whole house fan required in a California ADU?

Only in some cases. On the prescriptive path, the 2025 Energy Code requires a whole house fan in new single-family homes in climate zones 8 through 14, but new dwelling units of 500 square feet or less are exempt, and so are ADU additions of 1,000 square feet or less. Many ADUs don't need one, but a larger new detached ADU in an inland climate zone might.

Where the requirement comes from

Section 150.1(c)12 (ventilation cooling) says single-family homes must meet the whole house fan requirements shown in Table 150.1-A. That table marks the 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. The climate zone comes from your parcel address.

Exemptions that matter for ADUs

• Small new units: new dwelling units with 500 square feet or less of conditioned floor area don't have to meet the whole house fan requirement.
• Additions up to 1,000 square feet: Exception 5 to Section 150.2(a) exempts these additions from the ventilation cooling requirement. That covers most attached ADUs and garage conversions.
• Performance path: the prescriptive requirement can be traded off in a performance model. If the model does rely on a whole house fan, its airflow and fan efficacy must be field verified by an ECC-Rater.

If a fan is required

• Total fan airflow must be at least 1.5 cfm per square foot of conditioned floor area, using ratings from the Home Ventilating Institute Certified Products Directory.
• The attic needs at least 1 square foot of vent free area per 750 cfm of fan airflow, or more if the manufacturer specifies it, unless the fan vents directly outdoors.
• You must give the occupant a one-page sheet on how to operate the fan.

Whole house fan vs. IAQ ventilation

A whole house fan is not the same as the required indoor air quality ventilation. The whole house fan is a cooling strategy: it pulls cool evening air through open windows and exhausts hot attic air. The whole-dwelling IAQ fan runs continuously or on a schedule to dilute indoor pollutants, and new ADUs need it regardless of climate zone. A home can need both.

Design notes

• A whole house fan needs an attic or a direct path outdoors, so ADUs with vaulted or cathedral ceilings may find the performance path simpler.
• The fan's ceiling opening has to be coordinated with insulation, the attic access and the lighting layout.

What this means for your MEP plans and Title 24 report

The Title 24 report confirms your climate zone, project type and floor area, then shows whether a whole house fan is required, exempt or traded off. If one is required, the mechanical and electrical plans show its location, airflow and circuit, and the attic venting must match. See why your climate zone matters, Title 24 in climate zone 12, and the ADU Title 24 guide.

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