Does a garage conversion need HVAC?

Yes. A garage has no residential heating, cooling, or ventilation, so a garage-to-ADU conversion requires a new HVAC system designed to residential standards — most often a ductless mini-split heat pump — plus engineered whole-building ventilation. Both must be shown on permit-ready mechanical plans.

Heating and cooling: why a mini-split fits

To qualify as a legal dwelling, an ADU must maintain habitable temperatures, and any existing garage unit heater won't do it — those are non-residential heaters built for uninsulated, non-habitable space and are removed as part of the conversion. A single-zone ductless mini-split heat pump is the standard solution for a converted garage: it needs no ductwork, fits compact floorplans, and is strongly favored under California's 2025 energy code, which makes heat pumps the prescriptive baseline. Rough installed costs run about $3,500-$6,500 for a one-car garage and $7,500-$12,000+ for multi-zone systems in larger two-car conversions.

Ventilation: the step that's easy to miss

This is the part homeowners overlook. While the garage door was in place, the gap around it provided substantial unintended air exchange. Once that opening is framed and sealed into a finished wall, that air exchange disappears, and a properly designed whole-building mechanical ventilation system meeting ASHRAE 62.2 must be added and documented. The mechanical plan also covers bathroom exhaust, kitchen exhaust, and condensate drainage.

How HVAC ripples into the rest of the plan set

• Electrical — the mini-split needs a dedicated circuit, sized into the load calculation and panel schedule.
• Title 24 — a new heat pump triggers HERS refrigerant charge verification, and the equipment efficiency on the CF1R must match the mechanical plans exactly.
• Fire separation — refrigerant lines or ducts that penetrate a fire-rated wall between the ADU and the main house must be fire-stopped, and that's shown on the plans.

Confirm whether your city has adopted an all-electric reach code — many California cities prohibit gas appliances in ADUs, which makes an all-electric heat pump the required path. Requirements vary by jurisdiction; confirm with your local building department.

Sizing it right — not by rule of thumb

Mini-split capacity isn't picked by floor area alone. The right size depends on the conversion's insulation, window area and orientation, ceiling height, and your climate zone — the same factors the Title 24 model uses. An oversized unit short-cycles and dehumidifies poorly; an undersized one can't hold temperature on design days. Because making the garage habitable usually involves upgrading insulation anyway, the envelope improvements and the HVAC sizing are best worked out together, which is another reason the mechanical plan and the energy report come from one coordinated effort.

What the mechanical sheet shows the inspector

• The HVAC equipment schedule — indoor and outdoor unit make/model, capacity, and efficiency ratings
• Refrigerant line routing and condensate drainage
• Whole-building ventilation per ASHRAE 62.2, with the fan CFM specified
• Bathroom exhaust and kitchen exhaust, with fan CFM
• Fire-stopping details for any penetrations through a rated wall

These are the items the inspector compares against the installed work at rough mechanical and final, so getting them complete and accurate up front is what keeps inspections clean.

Our mechanical plans include the equipment schedule, heat-load basis, and ASHRAE 62.2 ventilation design reviewers expect, and bundle cleanly with electrical, plumbing, and Title 24. Start your order or see the Full MEP Package.