Do I need a mechanical plan for a garage conversion if the garage already has a heater?

Almost always, yes. Converting a garage into an ADU triggers the full set of mechanical requirements for a new dwelling unit — and an existing garage heater rarely satisfies them. The conversion creates habitable space, so the code now requires a permanently installed system that maintains 68°F in every habitable room, whole-building ventilation, and Title 24 compliance, none of which a typical garage heater provides.

Why an existing heater usually doesn't count

A garage was never habitable space, so any heater already there was installed under a different, lower standard. When the garage becomes an ADU it must meet the requirements for a dwelling unit:

• Permanently installed heating to 68°F in every habitable room. The California Residential Code requires heating capable of maintaining at least 68°F at a point 3 feet above the floor and 2 feet from exterior walls in all habitable rooms. A single radiant garage heater or unit heater generally cannot demonstrate this across the new floor plan, and many such heaters are gas-fired units that don't meet current standards or local reach codes.
• Whole-building ventilation. Title 24 requires continuous ASHRAE 62.2 mechanical ventilation in the new unit — something garages never had.
• Cooling, effectively. The code mandates heating, not cooling, but in inland and desert climate zones cooling is essential for occupant safety, which a heater alone can't provide.
• Title 24 compliance. The conversion is new conditioned floor area and must be modeled, with equipment meeting current efficiency minimums.

What the conversion typically needs

In practice, most garage-conversion ADUs replace the old heater with a ductless mini-split heat pump, which solves heating, cooling, and all-electric compliance in one unit, and adds a dedicated ASHRAE 62.2 ventilation system. The mechanical plan documents:

1. A Manual J load calculation for the converted space (garages often have minimal insulation, which changes the load — the engineer applies code-minimum or upgraded envelope values).
2. The heat-pump equipment schedule, sized to that load and meeting Title 24 minimums.
3. The condenser location with property-line and window setbacks.
4. ASHRAE 62.2 whole-building ventilation plus bath and kitchen exhaust ducted to the exterior.
5. Condensate routing and refrigerant line-set routing.

Garage conversions also usually require envelope upgrades — insulation, sometimes the slab and walls — to become habitable. Those upgrades change the heat load, so reusing the old heater without a load calculation is rarely defensible at plan check.

The narrow exception

If — and only if — the existing system happens to be a permanently installed, code-compliant heat pump or furnace already sized for the converted space and meeting current Title 24 efficiencies, it might be retained. That is uncommon for a garage heater, and even then a mechanical plan is still needed to document sizing, ventilation, and compliance for the permit.

Garage-conversion requirements and envelope rules vary by jurisdiction — confirm with your local building department before assuming any existing equipment can stay. Our mechanical plans for garage conversions include the Manual J, equipment schedule, and ventilation design sized to the converted space, and pair with a Title 24 report. See our mechanical guide or start your order.