What is the total cost to drywall and finish a new addition on a Toronto semi-detached house?

The total cost to drywall and finish a new addition on a Toronto semi-detached house typically ranges from $8,000 to $25,000 for the drywall scope, depending on the size of the addition, ceiling height, finish level, and whether fire-rated or soundproof assemblies are required on the shared party wall. A typical rear or side addition of 300 to 600 square feet of floor area generates 1,200 to 3,000 square feet of wall and ceiling surface that needs drywall.

Semi-detached homes have a critical requirement that standalone houses don't — the shared party wall between your home and the neighbouring unit. Ontario Building Code requires this fire separation to achieve a minimum 1-hour fire resistance rating and an STC (Sound Transmission Class) rating of 50 or higher. If your addition extends or modifies the party wall in any way, the new section must meet these standards. This typically means 5/8-inch Type X fire-rated drywall on resilient channel (to decouple the drywall from the framing and reduce sound transmission), with all joints properly taped and finished. The fire-rated assembly on the party wall runs $3.50 to $6.00 per square foot — significantly more than standard wall drywall at $2.50 to $4.00 per square foot.

For the exterior walls of the addition, the drywall goes over insulated framing with a 6-mil polyethylene vapour barrier on the warm side, as required by Ontario Building Code for Climate Zone 6. The insulation must meet a minimum R-24 for above-grade walls in renovations. Drywall installers should verify that insulation and the vapour barrier are properly installed before any board goes up — once drywall covers the walls, deficiencies in the vapour barrier or insulation are invisible but can cause serious moisture and mould problems down the line.

Ceiling drywall in an addition runs $3.50 to $5.50 per square foot using 5/8-inch board (recommended for ceilings to prevent sagging). If the addition includes a vaulted or cathedral ceiling, expect the price to increase by 25 to 40 percent due to the difficulty of working at angles and heights. Flat ceilings with pot lights or other fixtures require careful cutting and finishing around each opening.

The interior partition walls within the addition (bedroom walls, bathroom walls, closets) use standard 1/2-inch drywall at $2.50 to $4.00 per square foot for hanging. Bathroom and laundry areas should use moisture-resistant green board or mould-resistant purple board — green board runs $20 to $28 per 4x8 sheet compared to $14 to $20 for standard board, adding roughly $0.50 to $1.00 per square foot to the material cost.

Taping and finishing across the entire addition adds $2.00 to $3.50 per square foot for a standard Level 4 finish. For an addition with large windows (common in Toronto rear additions designed to bring in natural light), strongly consider a Level 5 finish at $3.00 to $5.00 per square foot for walls that receive direct sunlight — the raking light will expose every joint line in a Level 4 finish. Open-concept additions where the new space flows into the existing home are especially demanding because the eye naturally compares the new walls to the existing ones.

What Affects the Final Price

Several Toronto-specific factors influence your addition's drywall costs. Access and staging matter — semi-detached homes on narrow Toronto lots (many are only 15 to 20 feet wide) make material delivery and debris removal challenging. If drywall sheets can't be brought in through a side door or window, they may need to come through the house, adding labour time. Matching existing textures where the addition meets the original home requires skill — if your existing walls have a knockdown or orange peel texture, the finisher needs to blend the new work seamlessly into the old at $1.50 to $3.00 per square foot for texture application.

Permits are mandatory for any addition, and the building inspector will want to see the framing, insulation, vapour barrier, and fire-rated assemblies before drywall goes up. Don't let your contractor cover anything until the rough-in inspection is passed — ripping out drywall to fix deficiencies found later is expensive and demoralizing.

A new addition is firmly in professional territory. You need a drywall crew experienced with fire-rated party wall assemblies, vapour barrier detailing, and achieving seamless transitions between new and existing spaces. Browse drywall and insulation contractors through the Toronto Construction Network directory at torontoconstructionnetwork.com/directory?trade=insulation for free estimates on your project.