Every few months a homeowner calls a planning department asking why their garden-suite building permit is complicated when "Bill 23 made it legal." The short answer: Bill 23 made it as-of-right — meaning you don't need a rezoning — but it didn't remove building permits, OBC compliance, lot coverage caps, setbacks, or spatial-separation rules. You still have to build it correctly, and "correctly" is a layered question. Here is what each layer means in practice.
The More Homes Built Faster Act, 2022 amended the Ontario Planning Act to require municipalities to allow up to three residential units per residential lot as-of-right — that is, the primary dwelling plus up to two Additional Residential Units (ARUs). Before Bill 23, a municipality could refuse a garden-suite permit simply because the by-law didn't contemplate one — you'd need a rezoning application (6–18 months, $3,000–$15,000 in fees, public notice, Committee of Adjustment) just to get to the point where you could apply for a building permit. After Bill 23 — and specifically after municipalities updated their by-laws, which they were required to do by January 1, 2024 — you submit a building-permit application and the planning department cannot refuse it solely on the ground that the lot already has a house. The as-of-right right is the right to apply without a rezoning. It is not a right to build anything you want anywhere you want at any height or density you want.
Three units is the provincial maximum for as-of-right permission. The three can be arranged several ways: the most common for a standard urban lot is a primary dwelling + a basement secondary suite + a garden suite. But they could also be a duplex (two stacked or side-by-side units in one building) + one detached ARU. What matters for this tool is how many units already exist on the lot. If someone added a secondary suite five years ago, the lot now has two units; you have room for one more ARU, not two. Many title searches in Ontario will now flag a secondary-suite permit in the history — that's the first place to check before designing a garden suite.
A secondary suite (basement apartment, in-law suite) is a self-contained unit inside the primary building. The critical fire-safety requirement is an internal fire separation between the two dwelling units, governed by OBC 9.10.9 — typically a 30-minute fire-resistance-rated floor/wall assembly, smoke-tight barriers, interconnected smoke alarms, and specific escape path requirements. The structure shares the primary building's foundation, roof, and building envelope.
A garden suite is a separate building in the rear yard. It has its own foundation (which must reach below the frost line), its own building envelope, its own roof, and its own fire-safety relationship with the adjacent buildings. Because it's a separate building, the fire-safety concern is not internal fire separation but spatial separation — the distance between the two buildings and what it implies for radiation risk, combustible cladding, and glazed openings on the facing wall. This is governed by OBC 9.10.14 (Spatial Separation Between Buildings) and OBC 9.10.15 (Spatial Separation Between Houses). The wall of the garden suite facing the primary dwelling — and the wall facing the property line — are both "exposing building faces" subject to the table-driven limits on glazed openings and cladding type.
The most common surprise in garden-suite design is the window situation on the wall facing the main house. Here's the physics: if two buildings are close together and one catches fire, the wall of the other building that faces the fire receives radiant heat. If that wall has a lot of glass — which has zero thermal mass — the glass heats up, fails, and the building ignites. Table 9.10.15.4 of the OBC quantifies this: for a house, the maximum percentage of the "exposing building face" that can be unprotected glazed openings depends on the limiting distance (the distance from the wall to the property line, or to the imaginary midpoint between the two buildings on the same lot). The table columns run from <1.2 m to 25 m. The critical thresholds:
The practical design implication: if you site the garden suite at 1.5 m from the main house, the wall facing the house can have very limited glazing — probably one small window, not a full glass wall. Site it at 3 m or more and the restriction effectively disappears for typical suite sizes. This drives garden-suite layout more than any setback rule. Use the Spatial Separation Calculator to run the exact table lookup for your wall area and distance.
Setbacks for the garden suite itself are set by your municipal zoning by-law — the OBC does not set them directly (aside from the spatial-separation constraints discussed above). Common Ontario municipal setbacks for garden suites: rear lot line 0.6–1.5 m; side lot lines 0.6–1.2 m. Some municipalities allow a garden suite to sit on the property line where it replaces an existing fence, subject to OBC fire-wall requirements (which then requires a full 1-hour fire-rated wall with no openings). Lot coverage — the percentage of the lot area covered by buildings — is typically capped at 35–45% in Ontario residential zones, and every existing structure on the lot counts toward that cap before you add the garden suite's footprint. A bungalow with a detached garage and a backyard deck can easily be at 30–35% coverage before the garden suite is added; on a smaller urban lot, coverage can become the binding constraint. The ToolFluency checker computes coverage from your inputs and flags it against the typical Ontario residential range.
A garden suite is a permanent building. It cannot be on skids or blocks — it requires a proper foundation that bears below the frost line, per OBC 9.12.2.1.(1) (excavations must reach below frost depth) and OBC 9.15.3.1.(1) (footings must bear on undisturbed soil or rock below frost line). Ontario frost depths run from about 1.2 m in the Windsor/Niagara/Hamilton area to 1.4 m in the Toronto/Ottawa belt to 1.8 m in northern Ontario cities. The depth for your specific site is confirmed by the building department using the 2.5% January design temperature from OBC Supplementary Standard SB-1 climate data for the nearest weather station. Foundation options include: a full poured concrete perimeter wall, an ICF (Insulating Concrete Form) foundation for better insulation, helical steel piers driven below frost (fast, minimal excavation, excellent for small structures), or a preserved wood foundation. A concrete pad-on-grade is NOT an acceptable foundation for a garden suite unless the soil is engineered and thermally protected against frost heave — this requires a design engineer and is rarely cheaper than a proper footing. Budget the foundation at $15,000–$35,000 depending on depth, soil conditions, and system type. The Frost Depth Calculator and Deck Footing Depth tool give precise values by climate zone.
Ontario Reg 299/19 explicitly permits a garden suite to share water and sewer service with the primary dwelling — you do not need a separate service to the municipal main. In practice, shared service is the most common and most cost-effective approach. A plumber extends the supply line from inside the primary house to the garden suite and connects the garden-suite drain to the existing sewer stack or an exterior cleanout. A sub-meter may be required by the municipality if they want to track water use per unit. Separate service is required when: the existing lateral is corroded or undersized (common in houses built before 1960 with clay tile sewer pipes), the municipality mandates separate service for a second metered unit as part of their infrastructure development-charge regime, or the lot is long enough that a separate service is cheaper than trenching the full length. On rural lots with a private well and septic system, the garden suite can share the well pump but the existing septic bed must be sized for the additional daily sewage load (typically 170–200 L/person/day, regulated by the local health unit and OBC Part 8). An undersized bed will require either a new larger bed, a replacement tertiary-treatment unit, or a holding tank — a significant cost that should be assessed before you design the garden suite. Consult a BCIN Class M designer for any septic sizing question.
A new garden suite built by a registered Tarion builder on a lot where it is the first home or the first new unit being constructed may trigger Tarion new-home warranty requirements — verify with your builder and Tarion. Most garden suites built as renovations to an existing lot by the homeowner or a renovation contractor are not subject to Tarion's new-home warranty program, but the OBC building permit process requires inspections (foundation, framing, insulation, final). Financing a garden suite can be done through a HELOC on the primary property (most common), a construction mortgage, or a rental-income-backed loan if the suite is pre-leased. Conventional appraisals for the primary property will often include value for the legal additional unit — a properly permitted garden suite can add $80,000–$200,000 to the appraised value of the property in a major Ontario market. Insurance: notify your home insurer before construction. An additional dwelling unit — even under construction — may affect your homeowner's policy. Most insurers will add a rider for a rental unit; some require a separate landlord's policy for the garden suite once it is occupied.
A garden-suite building permit in Ontario typically requires: a site plan drawn to scale showing all existing and proposed structures, all setbacks dimensioned to property lines, the lot area and coverage calculation, the existing septic/well location if rural; architectural drawings showing floor plan, elevations, sections with ceiling heights (must meet OBC 9.5.3 minimums — 2.4 m is standard), and egress windows from sleeping areas (OBC 9.9.10 and 9.9.9); structural drawings (foundation, floor joists, rafters or trusses) with the species and grade, span calculations or references to the OBC span tables in Section 9.23; an energy compliance form (SB-12 for energy efficiency); and the BCIN number of the designer (Division C 1.3.1). Most municipalities turn around a single-family-class permit in 10–30 business days under the OBC's prescribed timelines. The permit fee is typically $10–$20 per $1,000 of construction value — a $200,000 garden suite costs $2,000–$4,000 in permit fees. Inspections happen at: foundation before pour, framing/rough-in, insulation before drywall, and final occupancy.
Verified against Ontario Building Code 2024 (January 16, 2025 amendment) — Division B 9.10.9, 9.10.14, 9.10.15, 9.12.2.1, 9.15.3.1, 9.27; Ontario Planning Act s.16(3) as amended by Bill 23 More Homes Built Faster Act, 2022; Ontario Reg 299/19 (Additional Residential Units) as amended 2024. Setback, lot-coverage, and municipal-adoption inputs are based on planning policy — not OBC-published values. Frost depth estimates are approximations — verify with your local building department. Last verified 2026-06-01 by a Red Seal Carpenter.
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