What is the average cost to drywall a Toronto home office addition built over an existing garage?

Drywalling a home office addition over an existing garage in the GTA typically runs $8,000–$18,000 for the complete drywall scope, depending on the size of the addition, finish level, soundproofing requirements, and whether fire-rated assemblies are needed between the garage below and the new living space above.

Here's how that number breaks down and what drives it.

The Fire Separation Question Changes Everything

The single biggest cost variable in this project is the floor/ceiling assembly between the garage and the new office above. Ontario Building Code requires a minimum 45-minute fire resistance rating between an attached garage and any living space — and that rating applies to the ceiling of the garage (which is the floor structure of your new office). This means 5/8-inch Type X drywall on the garage ceiling side, with all joints properly taped and finished, and any penetrations (electrical, plumbing, HVAC) fire-stopped. If your contractor skips this or installs regular 1/2-inch board, the fire separation is non-compliant and your building permit inspection will fail. Budget $1,500–$3,500 for the garage ceiling fire-rated assembly alone, depending on the square footage and complexity of penetrations.

The walls of the office addition itself are typically standard 1/2-inch drywall on exterior walls and interior partitions, but the exterior walls need a 6-mil polyethylene vapour barrier on the warm side before boarding — this is Ontario Building Code for Climate Zone 6 and non-negotiable in a heated living space. Your framer or insulation contractor should install this before the drywall crew arrives, but verify it's done. Missing vapour barrier in a garage addition is one of the most common causes of mould problems in GTA renovations.

Breaking Down the Full Cost

For a typical 400–600 square foot home office addition over a garage, here's what to expect:

Walls and ceiling hanging at $3.00–$5.50 per square foot for ceilings and $2.50–$4.00 for walls adds up quickly when you account for all surfaces — four exterior walls, any interior partitions, the ceiling above, and the garage ceiling below. A 500 sq ft addition with 9-foot ceilings has roughly 1,200–1,500 square feet of drywall surface when you include all walls and ceilings.

Taping and finishing is where the cost climbs based on your finish level expectations. A home office is a workspace where you'll likely have task lighting, monitors, and potentially a window wall — all of which create raking light that exposes imperfections. A Level 4 finish (the GTA standard for painted walls) runs $2.00–$3.50 per square foot. If you're planning a smooth, high-end look with large windows or pot lights on a flat ceiling, budget for Level 5 at $3.00–$5.00 per square foot — the extra skim coat is worth it in a space with critical lighting.

Materials for a 500 sq ft addition — boards, compound, tape, corner bead, screws, and primer — typically run $1,800–$3,500 depending on board type and finish level selected.

Putting it together for a 500 sq ft addition: $10,000–$15,000 is a realistic all-in drywall budget (materials, hanging, taping, finishing to Level 4, and the garage ceiling fire-rated assembly). Larger additions or Level 5 finish push toward $15,000–$18,000.

GTA-Specific Considerations

This type of project almost always requires a building permit — you're creating new habitable living space, which triggers full Ontario Building Code compliance including structural review, insulation requirements (minimum R-24 for above-grade walls), fire separation, and electrical inspection. Don't let any contractor suggest skipping the permit on a garage addition. The permit process also means a building inspector will review the drywall fire separation before it's covered, which is actually your protection as a homeowner.

Seasonal timing matters for this project. If the addition is being drywalled in winter, the space must be heated to a minimum of 10 degrees Celsius throughout the taping and drying process — joint compound that freezes before curing is destroyed and must be removed. Temporary propane or electric heat adds $200–$600 to winter projects but is essential. Summer work in an unventilated addition can slow drying times significantly; your contractor should plan for adequate airflow.

Soundproofing between the garage and office is worth considering at rough-in stage. If you plan to use the garage below for vehicles, woodworking, or any noisy activity, adding resilient channel and a second layer of 5/8-inch drywall on the garage ceiling (in addition to the fire-rated layer) dramatically improves sound isolation. This is far cheaper to do during construction than to retrofit later — budget an additional $1,500–$3,000 for a proper sound-isolation assembly on the garage ceiling.

Need help finding a drywall contractor experienced with garage additions and fire-rated assemblies? Toronto Drywall Installers can match you with local professionals for a free estimate — or browse the Toronto Construction Network directory at torontoconstructionnetwork.com for drywall and related trades in your area.