Ontario Building Code Part 11 is the renovation chapter nobody reads until they're standing in a gutted bathroom wondering why the inspector wants new exterior insulation. The Code does give you meaningful relief when you renovate — but the relief is scoped, not unlimited, and one wrong move on a scope sheet drops you from Basic Renovation into Extensive Renovation or into Part 10 Change of Use. Here is the landscape from someone who has walked it.
This is the single biggest thing people get wrong. The Ontario Building Code does not grandfather existing buildings indefinitely. What OBC 11.4.1.1 says — the most important sentence in Part 11 — is that the performance level of the building after construction cannot be less than the performance level before. You keep what you have. You do not automatically get to keep it once you start tearing into the house. The moment you materially alter the building (11.1.2.1), Part 11 is triggered. Any new building system or extension has to comply with all other Parts of the Code (11.3.1.2). Any substantial removal of walls, ceilings, floors, or roof assemblies trips you into Extensive Renovation (11.3.3.2), where the reconstructed elements must fully comply with current Code. The scoped relief is real — you do not have to tear out the rest of the house to renovate one room — but it ends at the edge of the work zone. The rest of the house keeps its existing performance level, not its exemption from the Code.
Basic Repair is the lightest category — like-for-like replacement with the same or equivalent materials. The Code language in 11.3.3.1 covers "reuse, relocation or extension of the same or similar materials or components." Paint, flooring, cabinet swaps, shingles over existing sheathing, a furnace on the same lines. No code upgrade on the rest of the building. Basic Renovation is 11.3.3.1 with more ambition — removing a non-bearing wall, adding a bedroom, finishing an open basement, replacing a cladding assembly. The RE-BUILT assembly must meet current code (9.25 air/vapour barrier, 9.36 + SB-12 insulation, 9.9.10.1 egress where a bedroom is involved), but the rest of the building keeps its existing performance level. Extensive Renovation under 11.3.3.2 is the gut job — when interior walls, ceilings, floors, or roof assemblies are substantially removed and new ones installed, the structural and fire-resistance elements of the reconstructed areas must fully comply with all other Parts. The Code does not set a fixed percentage for "substantial" — inspectors call it when the framing pattern does not survive the demo. Change of Use under Part 10 is a different animal (covered below). New Construction is full Code — what the NEW portion of an addition must meet under 11.3.2.1.
Here is how a simple scope turns into a permit package. Homeowner says: "I'm finishing my basement and I want one bedroom down there." That single word — bedroom — pulls in 9.9.10.1: the room needs an egress window with ≥0.35 m² clear opening, no dimension under 380 mm, sill no higher than 1 m. If the existing window is smaller, you're enlarging the rough opening (structural header, full permit, possibly a window well per 9.9.10.1.(5)). 9.10.19 then triggers hard-wired, interconnected smoke alarms on every storey and in every bedroom — not just the new one. 9.33.4 requires a CO alarm within 5 m of sleeping areas where there's a fuel-fired appliance. Table 9.32.3.3 may bump your principal ventilation fan up because the rate scales by bedroom count. If the bedroom is in a new secondary suite, 9.10.9.14 demands a 45-minute fire separation between dwelling units plus STC 50 under 9.11.1. One bedroom. Five clauses. Every one gets inspected.
The like-for-like HVAC swap is Part 11 Basic Repair in its purest form — no permit, no code upgrade on the rest of the system. But the moment the scope widens, the relief narrows. Add or relocate ductwork and you're in 9.32 mechanical-permit territory. Install the first gas appliance in a building that was previously all-electric and you trip 9.33 and a TSSA gas permit. Switch fuel source — oil to gas, electric to gas — and you need venting, combustion air, and gas line all designed under 9.33 plus TSSA. Even the heat-pump retrofit everyone is chasing under federal rebate programs can trip an ESA panel upgrade when the old house has 60-amp or 100-amp service and the heat pump needs 40-60 amps of new load. The Code gives relief on a swap of a metal box on the same lines; the relief disappears the moment the lines themselves change. Good rule: if the technician spends more than an afternoon on the install, call the inspector before drywall goes back up.
Part 10 is a completely separate compliance path, much stricter than Part 11. It kicks in any time the major occupancy classification changes — house to duplex, residential to commercial, attached garage to living space, house to retirement home. Under 10.3.1.1, the building has to meet current-code requirements for the NEW occupancy across egress, fire separations, early-warning, ventilation, HVAC, and structural capacity. Part 11's scoped relief does not extend to parts affected by the change of use. 10.3.2.2 triggers a performance-level evaluation — if the existing framing can't carry the new live load, you either beef it up or post load restrictions per 11.4.3.2. 11.4.2.5.(3) flags a trap many homeowners miss: adding bedrooms OR exceeding 15% of the finished area in a dwelling unit triggers a sanitary-sewage-flow check against 8.2.1.3 — your septic may not have capacity. On top of the OBC, every municipality has zoning rules about occupancy. Passing the Code test is not passing the zoning test. Change of Use always needs a BCIN designer or P.Eng on the file.
Here is where Part 11 bites most often. The moment you open a wall — for plumbing, wiring, structural work, or water damage — the RE-CLOSED wall has to meet current code, not the old R-12 batt in 2×4 studs. Current OBC 9.25 wants a continuous vapour barrier on the warm side. 9.25.3 wants a continuous air barrier sealed at every seam. OBC 9.36 with SB-12 2024 sets insulation thickness by climate zone — most of Ontario's Zone 6 now wants R-22 in the cavity OR R-17 cavity plus R-5 exterior. A 2×4 wall opened for plumbing can no longer simply close back with batt — you commonly need exterior rigid foam, mineral wool, or a switch to 2×6 to hit SB-12. Part 11 gives relief on walls you DON'T open (they keep their existing performance under 11.4.1.1), but the opened cavity sits on the Basic Renovation path and must meet current Code. Plan the insulation upgrade as part of the scope, not as a surprise at framing inspection.