An addition is new construction attached to an existing house, so it carries most of the costs of building (a foundation, framing, a roof, windows, insulation, mechanical systems) plus the work of tying into the old structure. The type of addition matters most: a single-storey bump-out costs the least, a second storey over the existing house the most, and converting space you already have (a garage or basement) usually costs less than building outward. This page goes through each type and what drives its cost. It is part of our directory of what things cost in Canada; building an entire house is covered in the cost to build a house.
Types of additions
| Type | What it involves |
|---|---|
| Single-storey bump-out | A new foundation, walls and roof extending a room |
| Second-storey addition | Building on top of the house; the existing foundation and structure must be checked and often reinforced, and the roof is removed |
| Garage conversion | Insulation, a raised floor, windows and heating in an existing structure |
| In-law suite | An addition or conversion with its own kitchen and bathroom, built to code as a secondary unit |
| Sunroom or three-season room | A lighter structure, insulated for year-round use or not |
What drives the cost
| Factor | Why |
|---|---|
| Size | Most costs scale with the floor area |
| Foundation | A full basement under the addition costs more than a slab or crawl space |
| Roofline | Tying a new roof into a complicated existing one adds framing and flashing work |
| Plumbing | A kitchen or bathroom in the addition adds drains and supply lines |
| Structure | Openings into the existing house often need engineered beams |
| The existing house | Older or heritage homes may need upgrades to wiring, plumbing or the foundation |
| Finish level | The same choices as any renovation, from builder-grade to custom |
| Design | Architectural or design drawings and an engineer’s review |
Permits and zoning
Every habitable addition needs a building permit, and the design must meet the municipality’s zoning rules before it can be approved:
| Requirement | What it controls |
|---|---|
| Setbacks | How close the addition can be to the property lines |
| Lot coverage | How much of the lot buildings can cover |
| Height | Especially relevant to second-storey additions |
| Heritage rules | Design limits in some older neighbourhoods |
A design that doesn’t meet zoning needs a variance from the municipality, which adds time and cost. Permit fees are set by each municipality, often based on the value of the construction.
Adding space vs moving
Moving to a larger home brings its own costs: the real estate commission on the sale (which is negotiable; an industry survey by Payotte put the average total seller cost in Ontario at 4.3% of the sale price), land transfer tax on the purchase, legal fees, moving costs, and the price difference between your home and a larger one. Where larger homes cost much more than smaller ones, an addition can be the cheaper way to get space; where the gap is small, moving often is. Closing costs on a purchase are covered in the closing cost guides.
Planning and paying
Additions run over budget more often than smaller projects because problems in the existing structure only show up once it’s opened, so a contingency and a fixed-price contract for as much of the work as possible are common protections; the home renovation budget guide covers both. Because an addition costs as much as a large loan, many owners pay for it with a home equity line of credit, a refinance or a construction draw; those options are compared in home renovation loans.
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Sources
The figures and rules on this page come from these sources, last checked against them on September 25, 2026. How we check facts.