This surprises most people. A new house is built once, in sequence, with clear access and no existing structure in the way. An addition is built into a house that is already standing: the crew works around what is there, ties new footings to old ones, cuts openings in load-bearing walls, matches existing floor levels that are rarely level, and does it all with restricted site access. The per-square-foot number covers a smaller area, so the fixed costs — permits, engineering, mobilisation, temporary protection — are spread thinner.
| Type | Per square foot | Typical 400 sf project | Main cost driver |
|---|---|---|---|
| Rear or side ground-floor addition | $350–$475 | $140,000–$190,000 | New foundation and roof tie-in |
| Second-storey addition | $425–$600 | $170,000–$240,000 | Structural upgrade below plus full roof replacement |
| Bump-out (under ~100 sf) | $500–$750 | $50,000–$75,000 | Fixed costs over a very small area |
| Garage conversion to living space | $200–$325 | $80,000–$130,000 | Insulating and heating an unconditioned shell |
Ontario 2026 ranges, finished. Excludes permits, development charges, HST, and any servicing upgrade.
A ground-floor addition needs its own footings, below frost — typically at least 1.2 m below finished grade in southern Ontario. That is straightforward when the ground is good and access is open. It becomes expensive fast in three situations:
Our underpinning and shoring cost guide covers the first case, and the excavation takeoff guide covers how the dig volume is actually measured.
Adding a floor puts load on a house that was not designed to carry it. Expect an engineer to require some combination of:
Those items routinely add $40,000 to $90,000 on top of the finished-area price, and they are the ones most often missing from an early quote. They are also all measurable off the structural drawings once you have them.
| Item | Typical Ontario range | Notes |
|---|---|---|
| Architectural drawings | $6,000–$20,000 | Scales with complexity, not just area |
| Structural engineering | $3,000–$9,000 | Required for almost any addition |
| Building permit | $1,500–$6,000 | Municipality-dependent |
| Committee of Adjustment (if needed) | $3,000–$12,000 | Only if you need a minor variance |
| Survey | $1,500–$3,500 | Usually required for the permit |
| Temporary accommodation | Varies | Second-storey work usually means moving out |
The pattern is consistent. Early quotes price the visible work — framing, drywall, finishes — and under-price or omit the structural work that makes the visible work possible. The specific items we most often see missing:
Once you have permit drawings, the quantities are all on the sheets: footing sizes and lengths, concrete volume, framing lumber, roof area, window schedule, and the beams the engineer specified. Give Me The Cost reads the set and returns those measured quantities division by division with an Ontario Building Code check, so you can compare a quote against what the drawings actually call for rather than against another quote. Start with what a quantity takeoff is if the term is new.
How much does a 500 square foot addition cost in Ontario?
At 2026 rates, roughly $175,000 to $240,000 for a ground-floor rear addition and $215,000 to $300,000 for a second storey, finished. Permits, engineering, drawings and HST are additional.
Is an addition cheaper than moving?
It depends on land transfer tax, realtor fees and the price gap to the house you would move to. In the GTA, Toronto land transfer tax alone on a $1.5 million purchase is roughly $50,000, so an addition often competes well on pure cost. The trade-off is months of disruption.
Do I need a building permit for a home addition in Ontario?
Yes. Any addition that increases floor area requires a building permit, and almost all require structural engineering. Some also need a minor variance from the Committee of Adjustment if they exceed zoning limits on coverage, height or setbacks.
Why is a second-storey addition so much more expensive?
The existing house has to carry the new load. That usually means new beams and posts down to the foundation, enlarged footings under them, and sometimes foundation reinforcement. The roof comes off entirely and the house needs weather protection for the duration. Those items alone commonly add $40,000 to $90,000.
How long does a home addition take in Ontario?
Design and permits typically take three to six months before anything is built. Construction runs four to eight months for a ground-floor addition and six to ten for a second storey, longer if a minor variance is needed.