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Additions & Extensions

Rear Extension in Ontario

Building out the back is the most common way Ontario homeowners gain main-floor space, usually to open the kitchen into a bigger cooking and dining area. A rear extension pushes your foundation and roofline into the backyard, so the project needs a building permit, proper drawings, and new footings dug below frost depth before framing starts. Done well, the finished room reads as part of the original house rather than something added on later.

Zoning sets the limits: rear-yard setback rules decide how far you can extend, and lot coverage caps how much of your property can be built on. Many properties fit a useful extension within the bylaw; tighter lots go to the committee of adjustment for a minor variance. The structural work centres on the opening between old and new — typically a steel or engineered wood beam, sized by an engineer, so the back wall of the house can come out cleanly and the two spaces flow together.

What this covers

  • Design and permit drawings, including the engineered beam at the tie-in
  • Zoning review of rear-yard setback and lot coverage
  • Excavation and footings below frost depth, with foundation walls or piers
  • Framing, roof, windows, and doors for the new shell
  • Removal of the existing rear wall and installation of the support beam
  • Electrical, heating, and any plumbing runs by licensed trades
  • Insulation and air sealing to current code
  • Interior finishing tied into the existing floor and ceiling lines

What affects the price

  • Depth and width of the extension and the roof design over it
  • Foundation choice — full basement extension, crawl space, or slab
  • Size of the structural opening into the existing house
  • Whether the kitchen moves into the new space, which adds services and finishes
  • Grade and drainage conditions at the rear of the lot
  • Minor variance requirements on tight lots

Every home is different, so we don’t publish flat prices — the quote is free and based on your actual job.

Rear Extension near you

We take quote requests across Ontario. Local pages:

TorontoOttawaMississaugaBramptonCaledonHamiltonVaughanMarkhamRichmond HillAuroraNewmarketGeorginaWhitchurch-StouffvilleKingOakvilleBurlingtonMiltonHalton HillsOshawaWhitbyAjaxPickeringClaringtonKitchenerWaterlooCambridgeLondonBarrieInnisfilBradford West GwillimburyCollingwoodSt. CatharinesNiagara FallsWellandGrimsbyFort ErieGuelphWindsorKingstonGreater SudburyPeterboroughThunder BayBrantfordBellevilleSarniaWoodstockStratfordOrangevilleCornwallNorth BaySault Ste. MarieChatham-KentOrilliaWasaga BeachMidlandNew TecumsethSt. ThomasKawartha LakesCobourgPort HopePrince Edward CountyQuinte WestLeamingtonOwen SoundBracebridgeHuntsvilleNiagara-on-the-LakeTimminsBrockville

Common questions

How far can I extend into my backyard?
Your municipality's zoning bylaw sets a minimum rear-yard setback — often around 7.5 metres, though it varies widely — plus a cap on total lot coverage. The buildable envelope is whatever space those rules leave over. A zoning review at the start of design tells you exactly what fits as-of-right and whether a minor variance is worth pursuing for a deeper extension.
Why do footings need to be below frost depth?
Soil in Ontario freezes and heaves. Footings placed above the frost line move with it, cracking foundations and racking door frames. The Building Code requires footings below local frost depth — roughly 1.2 metres across much of southern Ontario — so the extension stays put through freeze-thaw cycles. An inspector verifies the depth before any concrete is poured.
Will the extension feel like part of the original house?
That is a design decision made early. Matching floor levels, ceiling heights, flooring, and trim makes the join disappear, and a wide engineered opening between old and new keeps sightlines continuous. Exterior matching — brick, siding, roof pitch — matters for resale. Some owners deliberately contrast the extension instead; either way, the transition detail deserves real attention in the drawings.
Can I extend the basement under the new addition?
Yes, and it is far cheaper to do during the extension than to attempt later. A full-depth foundation gives you storage or future finished space below the new room. The trade-offs are more excavation, waterproofing, and underpinning where the new basement meets the old one. Crawl-space and slab-on-grade options involve less work but give up that area for good.

Ready to sort out your rear extension?

Tell us what you're planning — one room or the whole home — and get a free quote.

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