Before the numbers, a quick look at why homeowners go this route and what it entails in the Toronto context:
Many Toronto homes (especially older ones in areas like The Beaches, Riverdale, Leslieville, etc.) have limited lot depth or tight side setbacks. Adding outwards (rather than up) preserves yard space, keeps disruption more contained, and usually doesn’t require major foundation reinforcement (depending on the condition of your home).
It’s a way to create more functional space, e.g., a larger kitchen/dining area, a family room that opens to the backyard, a main-floor bedroom, or an accessible “aging-in-place” addition without moving neighbourhoods.
On the flip side: it still involves structural work (foundation, roof tie-in, drainage, possibly relocating services), zoning/permit hurdles (setbacks, lot coverage, tree protection), and often higher cost than a standard renovation.
So it’s a big decision, but one with major upside if done right.
Here are realistic cost ranges for a one-storey addition in the Toronto/GTA market, followed by a worked example.
According to one contractor guide: home extensions typically range $400–$500 CAD per square foot for Toronto.
A deeper dive shows: for ground-floor expansions (i.e., one-storey additions), estimates of $220–$380 CAD per sq ft appear.
Another source: typical addition projects between $350–$600+ CAD per sq ft, depending on size/materials/complexity.
On Reddit, homeowners estimate around $200–$300 CAD/sq ft for simpler projects (though these may be older and less finished).
Because every project varies, it’s wise to assume a range rather than a single number.
Let’s say you’re planning a 300 sq ft (≈ 28 m²) rear extension for kitchen/dining:
Low-end: 300 sq ft × $220 = $66,000 CAD
Mid-range: 300 sq ft × $350 = $105,000 CAD
Higher-end (premium finishes, tricky site): 300 sq ft × $500 = $150,000 CAD
For a larger addition, say 600 sq ft:
At $400/sq ft you’re looking at $240,000 CAD.
At $450/sq ft you reach $270,000 CAD.
These align with multiple market sources: e.g., one guide cites 600 sq ft costing between $240,000–$300,000.
Here are some of the major factors influencing cost:
Scope & Size: Larger additions benefit from economies of scale; very small ones often have higher cost per sq ft.
Foundation / Site Conditions: If you need to excavate, underpin, deal with poor soil or tree roots, or tie into an old structure, costs go up.
Materials & Finishes: Standard finishes (vinyl siding, laminate flooring) cost less; custom millwork, high-end windows or architecturally complex shapes raise it.
Structural Complexity: Extending a roofline, adding large spans, dealing with existing structural loads, heritage constraints or zoning variances all add cost/time.
Access & Neighbourhood Constraints: In inner-city Toronto, narrow lots, proximity to neighbours, traffic/parking constraints, tree protection, etc., all may add cost.
Professional Fees & Permitting: Architecture, engineering, permit fees, zoning applications, tree/heritage reviews can add 10-20 %+ of the build cost.
Contingency & Time: Whatever the quote, plan a contingency of 10-20% because hidden issues often arise (older foundation, hidden utilities, etc.).
Since you asked about one storey, it’s worth comparing that to adding upward:
A one-storey addition typically does not require full foundation reinforcement (unless the existing foundation is weak) compared to adding a second storey, this can reduce complexity, disruption, and cost.
However, you are investing new roof, exterior walls, possibly new mechanical systems and a full foundation or slab depending on design.
According to one source, second-storey additions in Toronto cost approx $312–$390 CAD/sq ft (unfinished) or $360–$432 CAD/sq ft (turnkey).
So in many cases, a one-storey addition can cost somewhat less per square foot than a full second-storey build, but the actual cost depends heavily on site and finishes.
Here’s a checklist of typical cost items and things to budget for:
Permits and zoning (building permit, zoning review, tree protection, lot coverage relief if needed)
Architectural & structural drawings, engineering reports
Site prep & demolition (removal of existing structure, grading, soil removal)
Foundation/slab/piles (depending on ground conditions)
Structural framing (walls, roof tie-in)
Exterior envelope (roofing, siding, windows/doors)
Mechanical, electrical, plumbing (new HVAC/furnace/ducts if needed, new wiring/outlets)
Interior finishes (drywall, flooring, paint, trim)
Kitchen/bath (if you’re including them)
Landscaping re-instatement, drainage, exterior walkways/patio tie-in
Contingency (10-20%)
Possible temporary relocation or living-in-reno considerations
Given your detail-oriented mindset, here are actionable tips:
Define your scope firmly up front — locked-in square footage, finishes list, structural assumptions. Change orders can blow budgets.
Use a trusted contractor with Toronto experience who understands local zoning, permitting, and logistics in narrow-lot neighbourhoods.
Check the foundation/structure early — an older home may need structural upgrades to tie in safely; discovering this late will add cost.
Choose finishes after setting budget — finishes are flexible and often where over-spend happens.
Set aside contingency — you’ll likely encounter unexpected conditions (hidden moisture, soil issues, scheduling delays).
Don’t forget soft costs — architecture, engineering, permit fees, possibly tree/heritage reviews.
Plan for neighbourhood/lot access — in Toronto, tight lots + neighbours + city rules = sometimes higher logistic costs.
Think resale value — while customizing makes sense, ensure your design still appeals broadly; major over-capitalisation may not pay off.
Communicate with your builder about timing — Toronto labour and material markets are busy; delays cost money.
Monitor budget vs progress — track spend vs milestones so you can adjust scope if things go off plan.
For a one-storey addition in Toronto in 2025, you’re realistically looking at somewhere in the ballpark of $300-$450 CAD per sq ft for a turn-key finished space — with lower and higher ends depending on finishes and complexity. For example, a 400 sq ft bump-out could cost ~$120,000 to $180,000+ depending on site conditions and finish level. If you keep it simpler, say standard finishes, straightforward site — you may hit the lower end of that spectrum.
Because you’ve worked with structural drawings and costing materials (you mentioned framing by floor, foundation walls etc), you know how much variation site to site can bring. Use these per-sq-ft benchmarks as a sanity check, but always get detailed quotes.
If you're looking to add one or multiple stories to your house, contact Grand Design Build for consultation and quote.