HVAC Options for ADUs

ADU heating and cooling has a clear winner, a solid runner-up, and a couple of options you'll be offered anyway. Here's the field.

The winner: ductless mini-split heat pumps. A compact outdoor unit paired with one or more sleek indoor wall heads. They heat and cool, run remarkably efficiently (which helps your Title 24 compliance), need no ductwork (precious in small spaces with limited ceiling cavity), and zone naturally — a head in the living area and one in the bedroom, each with its own remote. Installed cost for a typical ADU: $4K–$9K. This is the default for good reason; the vast majority of ADUs we build use them.

The runner-up: ducted mini-splits. Same heat-pump efficiency, but the indoor unit hides in a closet or ceiling and feeds short duct runs to each room. You pay more ($7K–$12K+) and sacrifice a bit of efficiency for invisible equipment and no wall heads. Worth it for design-sensitive projects and multi-room units where wall heads in every bedroom feel cluttered.

The budget options: PTACs and packaged units. Hotel-style through-wall units are cheap and fine for a small studio, but louder and less efficient — and looking like a motel is not what maximizes rent. Window units don't belong in a new dwelling you spent six figures on.

Skip: extending the main house's HVAC. Tempting, usually wrong — the house's system wasn't sized for the extra load, you can't control zones independently, and sharing systems between dwellings complicates everything from billing to repairs.

The one mistake to avoid: oversizing. Contractors habitually round up, but an oversized unit in a small, well-insulated ADU short-cycles — blasting, stopping, never dehumidifying properly. Insist on a Manual J load calculation (the HVAC version of right-sizing) rather than a guess. Small, tight buildings need less machine than intuition suggests.