Can my ADU share the main house's furnace and ductwork?

Usually not. The CEC's 2025 ADU FAQ says existing furnace ducts with a central return cannot be extended to heat an attached ADU conversion, because the California Mechanical Code doesn't allow return air from one dwelling unit to be sent into another through the heating or cooling system. Since an ADU is its own dwelling unit, in most cases it needs its own system.

Why the code draws this line

A forced-air system with a shared return pulls air from every space it serves, mixes it, and sends it back out. Between two separate homes, that moves odors, smoke, cooking byproducts and airborne contaminants from one household to the other. It also takes away each occupant's control over their own heating and cooling. The return-air restriction keeps the two units' air separate.

What about the addition exception for extending existing systems?

Section 150.2(a) has an exception that lets heating or cooling be extended from an existing system to serve an addition without upgrading the existing equipment to current standards, as long as the heating capacity meets the California Residential Code minimum. Another exception says that when ducts are extended, the existing and new ducts must meet the duct sealing and insulation rules for alterations. These exceptions are about Energy Code efficiency. They don't override the Mechanical Code rule against sharing return air between dwelling units. They're meant for adding a room to the same home, not for serving a second unit.

Common approaches for ADUs

• Ductless mini-split heat pump: the most common choice. It's compact, needs no duct run back to the house, and is a heat pump, which is the prescriptive baseline under the 2025 code.
• Small ducted heat pump: serves the ADU only, with its own supply and return.
• Separate whole-dwelling ventilation: new dwelling units, including ADUs that are additions, need their own whole-dwelling ventilation sized to the ADU's floor area. A junior ADU that is an addition is exempt from the whole-dwelling airflow requirement.

Check before you assume

Every heating system still has to keep the ADU's habitable rooms at the minimum temperature the Residential Code requires. Your building department makes the final call on any unusual layout. Ask before you design around a shared system.

What this means for your MEP plans and Title 24 report

The mechanical plan should show a dedicated system for the ADU, sized from a load calculation. It should also show separate ventilation, and it should never show ducts connecting the two units' returns. The Title 24 report models that dedicated equipment and lists the ECC verification it triggers. See mechanical plans, why mini-splits are the standard ADU choice, and the ADU Title 24 guide.

Sources: energy.ca.gov, energy.ca.gov, energy.ca.gov.