What are the ventilation requirements for a new ADU in California?

Every new ADU in California must include a continuous, whole-building mechanical ventilation system meeting ASHRAE 62.2 — incorporated into Title 24 — plus local exhaust at the bathrooms and kitchen. This is mandatory regardless of ADU size, is separate from heating and cooling, and is one of the most commonly missed items on incomplete ADU plans.

The two layers of required ventilation

• Whole-building (dilution) ventilation: a continuous, mechanically driven supply of outdoor air sized to the dwelling's floor area and bedroom count. This is the headline requirement and the one most often missed. It is not waivable by size.
• Local (spot) exhaust: bathroom exhaust fans and a kitchen range hood ducted to the exterior (recirculating hoods generally don't satisfy the California Mechanical Code). These are in addition to whole-building ventilation, not a substitute.

Typical ADU ventilation rates

The required continuous rate rises with floor area and bedroom count. As rough benchmarks:

The engineer calculates the exact target for your unit; these figures illustrate the order of magnitude.

How ADUs meet the requirement

Several approaches satisfy whole-building ventilation, and the mechanical plan must specify which one is used:

• Continuous exhaust: a quiet, energy-efficient bath fan rated for continuous duty, wired to run continuously. Simple and economical.
• Balanced ventilation with an ERV or HRV: an energy- or heat-recovery ventilator exchanges stale indoor air for fresh outdoor air while recovering most of the heat (and, for ERVs, moisture). The most comfortable and efficient option.
• Supply ventilation: ducting a measured amount of outdoor air to the HVAC air handler so incoming air is filtered and tempered.

Why ventilation is non-negotiable in a tight ADU

Modern ADUs are built tight to meet the energy code, so they no longer "breathe" through gaps the way old construction did. Without deliberate mechanical ventilation, moisture, CO2, cooking byproducts, and VOCs from finishes accumulate indoors. That is exactly the problem ASHRAE 62.2 solves — and it is why a HERS rater field-measures the installed ventilation airflow to confirm the system delivers the required CFM. A system that looks right on paper still has to perform in the field.

Ventilation checklist for ADU plans

1. Calculate the required whole-building rate from floor area and bedroom count.
2. Select a strategy (continuous exhaust, ERV/HRV, or supply) and specify equipment and CFM.
3. Confirm the system is wired for continuous operation with appropriate controls.
4. Add local exhaust: a kitchen hood ducted to the exterior and bathroom fan(s) — separate from whole-building ventilation.
5. Note HERS verification of ventilation airflow on the plans.

Ventilation rates and verification requirements vary by code cycle and jurisdiction — confirm current details with your local building department. Our mechanical plans include the ASHRAE 62.2 design, calculated rate, and equipment schedule, and pair cleanly with a bundled Title 24 report. See pricing or start your order.