How much does a rear addition cost on a semi-detached home in Old Ottawa South?

Old Ottawa South has many older semi-detached homes, and a rear addition is a common way to gain a bigger kitchen, a family room or more main-floor living space. There is no reliable neighbourhood-level price for this work, though. The cost depends on the drivers below, which is why itemized quotes for your own house are the only numbers worth relying on.

The shared wall. On a semi, the wall between you and your neighbour is a fire separation, and the addition must not compromise it. You can't bear loads on or attach to your neighbour's side of the wall without their consent and proper engineering. How the fire separation is extended or terminated at the new back wall needs careful detailing, and your designer will confirm the assembly. Talking to your neighbour early helps.

Zoning. Your zoning sets maximum height, lot coverage and setbacks, and on a narrow lot those limits often decide how far back and how wide you can build. Ottawa has two zoning by-laws in effect while appeals are resolved, so check your property's zone under both. If the design exceeds a rule, you can apply to the Committee of Adjustment for a minor variance, and the 2026 urban fee totals $3,759. Planning or legal help for the application is extra. Since November 21, 2025, a provincial regulation also permits a reduced minimum setback of 90 per cent of the zoning requirement, which your designer can check against your lot.

Foundation. Excavating next to an older foundation may need shoring or underpinning to protect the existing house. Footings on clay or poorly defined soils must be at least 1.2 m deep and not shallower than the depth of frost penetration, and on sensitive clay such as Leda clay the footing sizes must be engineered. A full basement under the addition costs more than a crawlspace, but it adds usable space.

Access. Without side-yard access, materials, equipment and debris may have to come through the house or over it. Crane lifts for beams, where to put the bin, and how the concrete pump and excavator reach the back yard all affect cost. Ask each contractor how they plan to handle access on your lot.

Design choices. An expanded kitchen with eating space, a family room with large windows or doors, or a combined great room each carry different costs, mostly through cabinets, glazing and structural openings.

Permit. The addition needs a building permit, and the fee is $12.75 per $1,000 of construction value (minimum $117) under the 2026 schedule.

To compare itemized quotes, browse contractors with semi-detached and infill experience in the Ottawa Construction Network directory.

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