Does a garage conversion ADU or JADU need a whole-house ventilation fan?

A garage conversion that creates a new ADU does need whole-dwelling mechanical ventilation, even if it is under 1,000 square feet. A Junior ADU (JADU) that is an addition to an existing building is exempt from the whole-dwelling ventilation airflow requirement, but it still needs kitchen and bathroom exhaust.

Where the confusion comes from

Section 150.2(a)1C of the 2025 Energy Code exempts two kinds of projects from whole-dwelling ventilation airflow:

• Additions that enlarge an existing dwelling unit by 1,000 square feet or less, such as a family room addition.
• JADUs that are additions to an existing building.

The same section then says new dwelling units that are additions to an existing building must have whole-dwelling ventilation, sized on the new unit's conditioned floor area. A garage conversion ADU is a new dwelling unit, so the 1,000 square foot exemption does not apply to it. The CEC's ADU training materials say it directly: all ADUs must meet indoor air quality requirements regardless of size, and JADUs are exempt from whole-building ventilation only.

A separate rule, Exception 5 to Section 150.2(a), exempts additions of 1,000 square feet or less from the whole-house fan (ventilation cooling) requirement. That is a different system, and the two are easy to mix up.

How much airflow

The starting point is Equation 150.0-B: total required ventilation in cfm equals 0.03 times the floor area plus 7.5 times (bedrooms + 1), with bedrooms counted as at least 1. For a 400 square foot studio conversion, that is 12 plus 15, or 27 cfm before any infiltration credit. Detached units, and attached units that do not share ceilings or floors with other dwelling units, may reduce the fan rate using the code's infiltration adjustment; vertically attached units may not.

What still applies to a JADU

• Local exhaust in each kitchen and bathroom under Section 150.0(o)1G, plus ECC verification of a vented range hood under Section 150.0(o)2.
• Any California Mechanical Code ventilation requirements. The CEC's ADU FAQ notes the CMC may apply and may reference Section 150.0(o), and points JADU questions to HCD.
• If your JADU is carved out of space that was already conditioned, it is an alteration rather than an addition, so confirm the ventilation approach with your building department.

Testing

Whole-dwelling ventilation airflow is field verified by an ECC rater (formerly HERS rater), and CEC training materials note the IAQ fan is verified on ADU additions every time.

What it means for your mechanical plan

• State whether the project is a new ADU, an addition to the existing home or a JADU, since that drives the requirement.
• Show the calculated ventilation rate, the fan or system that provides it and its controls.
• Show local exhaust for every kitchen and bathroom either way.

For ventilation strategies, see ADU ventilation requirements and garage conversion MEP plans.

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