Home addition cost in Montreal — 2026 breakdown

The Hampstead clients we built a 380-square-foot rear addition for last year asked us, three months in, why the budget hadn't moved an inch. We were 40% of the way through the build and 40% of the way through the dollars. They had braced for the runaway-budget horror story their friends warned about. The truth is that an addition built on real drawings, a real engineer, and a real permit doesn't run away — it just isn't cheap to start with. Here's what one actually costs in Montreal in 2026.

The per-square-foot reality

A finished, properly built home addition in Montreal runs $350–$600 per finished square foot in 2026. That's the all-in number, including foundation, structure, mechanical, finishes, and project management. It doesn't include design fees, permits, or landscaping repair.

A 400 sqft single-story rear addition lands $140,000–$240,000. A 600 sqft two-story side addition lands $245,000–$420,000. A 900 sqft full-height rear extension with a finished basement underneath lands $385,000–$650,000.

The price spread is driven by four variables more than anything else.

The four variables that move the price

Foundation work. A new foundation is the single largest line on most additions. Excavation, footings, foundation walls, waterproofing, and weeping tile run $28,000–$65,000 depending on depth and soil conditions. Pile foundations on poor soils (rare in Montreal, but present in some Pierrefonds and DDO lots) add $15,000–$40,000.

Connection to the existing house. Cutting an opening in the existing exterior wall and tying the new structure into the existing roof, floor, and mechanical systems is harder than building the addition itself. Budget $18,000–$45,000 for the connection scope alone.

Height and structure. A single-story addition with a flat roof is the cheapest. A two-story addition with a sloped roof matching the original house is 35–60% more per sqft. A cantilever or unusual roofline can add another 15%.

Mechanical capacity. If the existing furnace, electrical panel, and water heater can't carry the new load, you'll be replacing them. Panel upgrade, new furnace branch, new gas line, possibly a tankless water heater swap — together $8,000–$22,000.

Where the dollars actually go

For a typical mid-range $280,000 addition, the breakdown:

• 22% foundation and excavation ($61,600)
• 18% structure — framing, roof, sheathing ($50,400)
• 14% mechanical — plumbing, electrical, HVAC, gas ($39,200)
• 12% exterior cladding and roofing ($33,600)
• 10% interior finishes ($28,000)
• 8% project management and overhead ($22,400)
• 6% windows and exterior doors ($16,800)
• 5% connection work to existing house ($14,000)
• 3% permits, surveys, engineering ($8,400)
• 2% landscaping repair around foundation perimeter ($5,600)

What homeowners forget to budget for

After 25 years of building additions, these are the line items that catch first-time addition clients.

1. Re-siding part of the existing house. Even if you match the addition to the original exterior, the original siding has aged. A small re-side on the connection face — typical cost $4,500–$12,000.
2. Landscape replacement. The excavation footprint is bigger than the addition footprint. Expect to repair roughly 30% more lawn and garden than the addition occupies. $3,500–$15,000 depending on the original landscaping.
3. Property survey update. Many additions require a current certificate of location, which costs $1,200–$2,500 and takes 4–8 weeks.
4. Mechanical relocation inside the existing house. Sometimes the new addition load requires re-routing existing ducting or moving the panel. $3,000–$10,000.
5. City service connection upgrade. Larger additions sometimes require an upgraded water service from the street. The city does this work for $8,000–$18,000 plus engineering.

Permits and timeline

Most Montreal additions need:

• A municipal building permit (4–8 weeks for issuance)
• Architectural drawings sealed by an OAQ architect (required above $200,000 in declared value)
• Engineer's stamped structural drawings
• A new certificate of location after construction
• A heritage review if the property is in a heritage zone

Realistic timeline from first call to keys back:

• Months 0–2: design, architectural drawings, structural engineering
• Months 2–4: permit submission, board approvals (if condo), city review
• Months 4–8: foundation, framing, weather envelope
• Months 8–10: mechanical, insulation, drywall
• Months 10–12: finishes, exterior connection, landscape repair

So a full 12 months from idea to keys is realistic for a 400–600 sqft addition. Stretch by 2–3 months for two-story builds. Compress only on small, single-story projects with no foundation surprises.

What we tell every addition client

Three things, on the first site visit:

1. The cheapest square footage in your house is the one you already have. Before sketching an addition, walk through whether reconfiguring existing space (basement, attic, garage) gets you the same usable area at half the price.
2. The hardest budget conversation is at month three. If we find an unexpected soil condition or a structural surprise during foundation work, it goes on the table the same day. Honest mid-project conversations are easier than year-end ones.
3. Don't undersize for resale. A 280 sqft addition costs almost the same per sqft as a 360 sqft addition (the fixed costs are spread). If the budget supports a slightly larger build, take it.

FAQ

Can Revohouse handle the architectural drawings?

Yes — we have OAQ-registered architects on retainer and provide a single contract that includes design, permitting, engineering, and construction. Most clients find single-contract scope (design-build) easier to manage than separate design and construction contracts.

How much value does an addition add to a Montreal home?

A well-built addition typically returns 65–85% of cost at resale. Functional additions (additional bedrooms or a primary suite) return more than purely cosmetic ones (sunrooms, screened porches).

What's the smallest addition worth doing?

Below about 250 sqft, the per-sqft cost climbs sharply because foundation, mechanical tie-ins, and permits are largely fixed costs. The minimum economic Revohouse addition is typically 280–320 sqft.

Can we live in the house during construction?

Yes, for most additions. The connection to the existing house is sequenced so the existing envelope stays sealed until the addition is weather-tight. Expect 2–4 days of high-disruption work when the actual connection opening is cut.