What are California's requirements for heating in an ADU?

Every Accessory Dwelling Unit in California must include a permanently installed heating system capable of maintaining at least 68°F in all habitable rooms. This is a mandatory health-and-safety requirement under the California Residential Code that applies to every ADU regardless of size, type, or climate zone.

The code requirement, precisely stated

The California Residential Code requires that heating facilities be capable of maintaining a minimum room temperature of 68°F at a point 3 feet above the floor and 2 feet from exterior walls in all habitable rooms. This is the same standard applied to any single-family dwelling. A few important consequences flow from this language:

• The heat source must be permanently installed — portable space heaters, plug-in units, and fireplaces alone do not satisfy the requirement.
• Heating must reach every habitable room. A studio met by one wall unit is fine; a multi-room ADU must demonstrate that heat reaches each bedroom and living space.
• The system must be sized to the design heating load for your climate zone, which is why a Manual J load calculation increasingly accompanies the mechanical plans.

What systems are allowed

California does not prescribe a specific heating technology — it sets a performance standard. Acceptable systems include electric resistance heaters, gas furnaces (where still permitted by local code), ductless mini-split heat pumps, ducted heat pumps, and hydronic or radiant systems. In practice, the combination of local electrification ordinances and the 2025 Title 24 energy code has pushed mini-split and central heat pumps to overwhelming dominance in new ADU construction because they satisfy the heating mandate, deliver cooling in the same unit, and earn favorable scores under energy compliance modeling.

Heating is only part of the mechanical picture

Two related mandates frequently surprise homeowners who think only about heat:

• Whole-building ventilation. Title 24 requires continuous mechanical ventilation meeting ASHRAE 62.2 in every new ADU. This is separate from heating and from bath/kitchen fans, and it is not waivable by size.
• Cooling. The code mandates heating, not cooling — but in Central Valley, inland, and desert climate zones (zones 11–15), summer design temperatures make cooling effectively essential for occupant safety. Specifying a heat pump solves heating and cooling in one all-electric system.

Sizing checklist for ADU heating

1. Confirm the project's California climate zone by parcel address (not ZIP code).
2. Run a Manual J load calculation to establish peak heating BTU/hr.
3. Select equipment that meets or exceeds current Title 24 minimum efficiency for that system type.
4. Verify every habitable room is served — count zones/heads against rooms.
5. Add ASHRAE 62.2 ventilation as a separate, documented system.
6. Confirm whether your city's reach code prohibits gas appliances before finalizing the heat source.

Heating requirements, ventilation rates, and rebate programs vary by jurisdiction and utility — confirm current details for your address. Our mechanical plans document the heating system, load calculation, equipment schedule, and ventilation in a single permit-ready set. If you want energy compliance handled in the same pass, add a Title 24 report when you place your order.