What HVAC system is most commonly used in California ADUs?

The most common HVAC system in California ADUs is the ductless mini-split heat pump — a two-piece system with one or more indoor heads connected by refrigerant lines to an outdoor condensing unit that provides both heating and cooling without any ductwork. It has become the default because it is all-electric, highly efficient, duct-free, and the path of least resistance to Title 24 compliance.

Why mini-splits dominate ADU design

1. No ductwork. ADUs are small and ceiling space is precious. Eliminating ducts saves height, avoids attic-routing headaches, and removes duct leakage — historically one of the largest sources of HVAC energy loss and HERS test failures.
2. All-electric. Mini-splits contain no combustion, so they comply with the all-electric reach codes adopted by Los Angeles, San Jose, San Francisco, Oakland, and dozens of other California cities, and they model favorably under the 2025 energy code.
3. Zoning flexibility. A multi-zone system gives each room independent temperature control, so an occupied bedroom and an empty living room don't have to share a setpoint.
4. High efficiency. Quality mini-splits typically exceed California's minimum SEER2/HSPF2 thresholds by a wide margin, which improves the Title 24 compliance margin.
5. Climate fit. California's largely mild, Mediterranean climate is nearly ideal for heat-pump operation across most of the year.

How configurations match the ADU

The main alternative

On larger multi-bedroom ADUs, a ducted heat pump with a central air handler is the principal alternative — same heat-pump technology, but air is distributed through a compact duct network to registers in each room with a single thermostat and no visible heads. It needs room to route ducts and the ducts must meet sealing limits verified by a HERS rater. Gas furnaces, by contrast, heat but don't cool, require a separate AC system for California summers, and are prohibited outright in cities with all-electric reach codes — so they rarely make sense for a new ADU.

Code thresholds and local mandates vary by jurisdiction — confirm with your local building department. Our mechanical plans specify the exact system, sized to your ADU and climate zone, with the Manual J and equipment schedule included. See our mechanical guide for deeper detail, or start an order.