How much does it cost to add supply and return ducts for a Toronto home addition?

Adding supply and return ductwork for a home addition in the GTA typically costs $1,500 to $6,000, depending on the size of the addition, the distance from your existing furnace, and whether your current HVAC system has enough capacity to handle the extra load. This is one of the most commonly underestimated costs in home additions — many Toronto homeowners budget carefully for framing, insulation, and finishes but forget that the new space needs conditioned air to be comfortable year-round.

The first question any ductwork contractor will ask is whether your existing furnace and air conditioner have enough spare capacity to serve the addition. A Manual J load calculation for the new space determines how many BTUs of heating and tons of cooling it needs, and that number gets compared to your existing equipment's capacity minus the existing load. If your system is already maxed out — common in older Toronto homes where the original furnace was sized for a smaller footprint — you may need a furnace upgrade ($3,000 to $6,000) or a separate mini-split system for the addition ($3,500 to $7,000 installed), which eliminates the ductwork question entirely.

Assuming your existing system has capacity, the ductwork cost breaks down as follows. For a small addition like a sunroom or single bedroom (100 to 200 square feet), expect one to two supply runs and one return, costing $1,500 to $3,000 including trunk line extension, branch ducts, register boots, registers, and all connections. For a larger addition (300 to 600 square feet) such as a family room or primary suite, you will need three to five supply runs and one to two returns, costing $3,000 to $6,000. The supply duct sizing depends on the Manual D calculation — typically 5-inch or 6-inch round branches for individual rooms, fed from the existing trunk line via new takeoff fittings.

Return air is the part most commonly skipped or undersized in GTA additions, and skipping it creates serious problems. Without adequate return air, the new room becomes pressurized when the system runs, pushing conditioned air out through every crack and gap in the building envelope. The result is a room that is always too hot in summer and too cold in winter despite having supply registers. Every addition should have at least one dedicated return air duct sized to match the total supply airflow to that space.

The City of Toronto requires a building permit for home additions, and the mechanical portion of that permit covers the ductwork. Permit fees typically run $200 to $800. Your contractor will need to show duct sizing calculations and demonstrate compliance with Ontario Building Code Part 6 requirements for airflow, fire dampers at any fire-rated assembly penetrations, and make-up air if the addition includes a kitchen with a large range hood. Get matched with a ductwork contractor through the Toronto Construction Network to get quotes specific to your addition layout.