Table of Contents
- What is a Secondary Suite / ADU / ARU?
- Types of Additional Dwelling Units
- Why Add a Secondary Suite?
- Zoning & Provincial Rules (Ontario 2025)
- Ontario Building Code Requirements
- The Permit Process
- Construction & Key Build Stages
- Typical Costs
- London, Ontario — Specific Rules
- Frequently Asked Questions
What is a Secondary Suite / ADU / ARU?
A secondary suite is a self-contained dwelling unit within, or attached to, an existing residential property — with its own kitchen, bathroom, sleeping area, and entrance.
In Ontario, these units go by several names depending on context:
- Secondary Suite / Secondary Dwelling Unit (SDU) — a unit inside the main house
- Additional Dwelling Unit (ADU) — the general term used in provincial legislation
- Additional Residential Unit (ARU) — the City of London's preferred term
- In-law suite / basement apartment — common informal names
- Garden suite / Laneway house — detached ADU on the same lot
Types of Additional Dwelling Units
Basement Suite
Most common in Ontario. Located in the basement of the principal dwelling. Requires egress windows, fire separation, and min. ceiling height.
Main Floor Suite
A portion of the main floor converted into a self-contained unit. Often used for accessible living or in-law suites.
Above-Garage Suite
An apartment built above an attached or detached garage. Popular for generating rental income on properties with existing garages.
Garden Suite
A detached structure in the rear yard. Now permitted as-of-right on most Ontario lots under provincial legislation.
Laneway House
A detached ADU built along a rear lane or alley. Common in older urban Toronto neighbourhoods; increasingly permitted elsewhere.
Coach House
A unit above or beside a detached garage or carriage house, typically in the rear yard.
Why Add a Secondary Suite?
💰 Mortgage Helper
Rental income from a legal suite directly offsets mortgage payments. At $1,500/month, that's $18,000/year — significant for any homeowner's budget.
📈 Increased Property Value
Legal, permitted suites command a measurable premium at sale — and broaden the buyer pool (investors, owner-occupants, multi-gen families).
🏠 Multi-Generational Living
Provides independent living space for aging parents, adult children, or family members — without sacrificing privacy.
🔒 Financing Advantage
Buyers can include rental income from a legal suite in mortgage qualification — allowing them to qualify for a larger loan or better rate.
🏙️ Housing Supply
Secondary suites add to the rental housing supply without requiring new land or major infrastructure — a key policy goal in Ontario.
🌱 Garden Suite Potential
Larger lots (like 60×152 ft) can accommodate a future detached garden suite — a third unit and additional income stream.
Zoning & Provincial Rules — Ontario 2025
Ontario's housing legislation has dramatically expanded where ADUs are permitted — but local municipality requirements still apply.
| Legislation / Rule | What It Allows | Effective |
|---|---|---|
| Planning Act S.16(3) | Municipalities must permit secondary suites in all single-detached, semi-detached, and rowhouses | 2019+ |
| More Homes Built Faster Act (Bill 23) | Up to 3 units as-of-right on most residential lots province-wide; municipalities cannot prohibit | Nov 2022 |
| Strong Mayors Act provisions | Streamlined approval timelines; reduced ability for municipalities to reject ADU permits on planning grounds | 2022–2023 |
| City of London ARU Regulations | Local rules on setbacks, lot coverage, parking, design standards — updated May 2025 | May 2025 |
What Municipalities Can Still Regulate
- Maximum suite size (e.g., % of principal dwelling or absolute sq ft)
- Minimum lot size
- Setbacks from property lines
- Parking requirements
- Design standards (exterior appearance)
- Owner-occupancy requirements (in some jurisdictions)
What Municipalities CANNOT Prohibit
- Secondary suites in detached houses, semi-detached, rowhouses
- ADUs in principle (as-of-right under provincial legislation)
- Garden suites / detached ADUs on residential lots
- Up to 3 units total on a residential lot (provincial minimum)
Ontario Building Code Requirements for Secondary Suites
Ontario Building Code (OBC) Division B Part 9 governs construction of secondary suites in existing houses. Key requirements include:
| Requirement | Specification | Notes |
|---|---|---|
| Minimum Ceiling Height | 1.95 m (approx. 6'5") in habitable rooms | Lower in non-habitable areas (storage, utility) |
| Egress Window | Min. 0.35 m² openable area, 380mm min. dimension in any direction | Required in sleeping rooms; may require window well in basement |
| Fire Separation | Min. 30-minute fire resistance assembly between units | Often achieved with 5/8" Type X drywall on one side of stud wall/ceiling |
| Smoke Alarms | Interconnected hard-wired alarms in each unit and on each floor | CO detectors required near sleeping areas where fuel-burning appliances exist |
| Exit Requirement | Each unit must have a means of egress without passing through another unit | Separate entrance or direct exit path required |
| Kitchen | Must have a cooking facility, sink, food storage | Full appliances (stove, fridge) typically required |
| Bathroom | Toilet, sink, and shower or bathtub within the unit | — |
| Plumbing | Hot and cold water supply, drain connection to municipal sewer | Plumbing permit required; backwater valve may be required |
| Heating | Adequate heating to maintain minimum temperature | Separate HVAC or zoned heating for the suite |
| Electrical | Dedicated circuits; ESA inspection required | Panel upgrade may be required if existing service is insufficient |
| Insulation | Appropriate thermal resistance per OBC climate zone | Vapour barrier required on warm side of insulation |
The Permit Process
A building permit is legally required for any secondary suite in Ontario. Here is the typical process:
Zoning & Pre-consultation
Contact your municipality's Planning and Building divisions. Confirm your property's zoning permits a secondary suite, verify any applicable conditions (lot size, parking, owner-occupancy), and ask about the local permit submission requirements and timelines.
Hire a Qualified Designer
Engage a Building Code–registered designer, architect, or professional engineer to prepare permit drawings. Required documents typically include: floor plans (existing and proposed), sections showing ceiling heights, fire separation details, egress window specifications, a site plan, and notes referencing OBC articles. An engineer may be required for structural modifications (e.g., beams, load-bearing walls).
Submit Building Permit Application
Submit Form 1 (Application for a Permit to Construct or Demolish) along with all required drawings to your local Building Division. Pay applicable permit fees (typically $1,000–$3,000 for a secondary suite project in Ontario). The Building Division has a regulated timeframe to review applications (10 business days for simple projects).
Permit Issued — Construction Commences
Once the permit is issued, post the permit card on-site and keep approved drawings accessible. Begin construction in accordance with the approved drawings. Do not deviate from approved plans without submitting a revision.
Required Inspections During Construction
Book inspections at each required stage. Typical inspection stages for a secondary suite: (1) footings/foundation if new; (2) structural framing after rough-in; (3) rough plumbing; (4) rough HVAC; (5) insulation and vapour barrier; (6) fire separation assembly; (7) ESA electrical (separate from building permit). Do not drywall over rough-ins before passing inspection.
ESA Electrical Inspection
All new or modified electrical work requires an Electrical Safety Authority (ESA) inspection, separate from the building permit process. Engage a licensed electrician and arrange the ESA inspection through the Ontario ESA portal. An Inspection Certificate is issued upon passing.
Final Occupancy Inspection
After all trades work is complete and all inspections are passed, book the final inspection with the Building Division. Upon passing, a Final Inspection Completed Notice or Occupancy Permit is issued — confirming the unit is legal and can be occupied and rented.
Key Construction Elements
Structural Work
Basement suite renovations often require structural modifications: removing or reframing load-bearing walls, installing support beams (LVL or steel), underpinning to increase ceiling height, and installing window wells for egress. Structural changes require engineer sign-off.
Fire Separation
The most critical OBC requirement. A minimum 30-minute fire-resistance assembly must separate the secondary suite from the rest of the house. Typically achieved with: 5/8" Type X drywall on ceiling and walls, firestopping at all penetrations (pipes, wiring, ducts), and fire-rated doors with self-closing hardware at common access points.
Plumbing
Rough-in for bathroom (toilet, shower/tub, vanity) and kitchen sink. Tie into existing stack or run new DWV. Backwater valve installation is strongly recommended (and often required) for basement suites to prevent sewage backup.
Electrical
Dedicated panel or sub-panel for the suite. Kitchen circuits, bathroom GFCI, smoke/CO alarm interconnection. Panel upgrade from 100A to 200A may be required if the main house panel is at capacity. ESA inspection required separately.
HVAC
Separate heating zone for the suite or standalone heating (electric baseboard, ductless mini-split). Ensure adequate ventilation (HRV or bathroom exhaust fan on timer). Each unit must be able to maintain minimum temperatures independently.
Finishing
Insulation (min. R-20 walls, R-31 ceiling in Ontario climate zone), vapour barrier, drywall, flooring, kitchen cabinetry, bathroom tile, paint, trim, and lighting. Quality of finish directly impacts rental rates and tenant satisfaction.
Typical Costs — Secondary Suite in Ontario (2024–2025)
| Item | Estimated Cost | Notes |
|---|---|---|
| Architectural / designer drawings | $3,000 – $8,000 | Depends on complexity and revisions |
| Structural engineer | $1,500 – $5,000 | Required for beam/structural modifications |
| Building permit fee | $1,000 – $3,000 | City of London rates; varies by municipality |
| Demolition / prep | $2,000 – $8,000 | Removing existing finishes, framing prep |
| Structural work (beams, framing) | $5,000 – $20,000+ | Higher if underpinning is required |
| Egress window well | $2,000 – $5,000 per window | Window + well + waterproofing |
| Plumbing rough-in + fixture | $8,000 – $18,000 | Kitchen + bathroom |
| Electrical (panel + rough-in) | $6,000 – $15,000 | Higher if panel upgrade required |
| HVAC / mechanical | $4,000 – $10,000 | Mini-split or ducted zone |
| Insulation + vapour barrier | $3,000 – $7,000 | Spray foam or batt + poly |
| Drywall + fire separation | $5,000 – $12,000 | Type X, taped and mudded |
| Flooring | $3,000 – $8,000 | LVP or tile throughout |
| Kitchen (cabinetry + appliances) | $8,000 – $20,000 | Wide range by quality tier |
| Bathroom (tile + fixtures) | $5,000 – $12,000 | Standard finish |
| Paint, trim, doors, hardware | $3,000 – $6,000 | — |
| TOTAL (full gut renovation) | $60,000 – $130,000+ | Depending on scope, finishes, and site conditions |
| TOTAL (legalization only) | $30,000 – $65,000 | Where basement is mostly finished but unpermitted |
London, Ontario — ARU Regulations (2025)
The City of London updated its Additional Residential Unit (ARU) regulations in May 2025 to align with Ontario's More Homes Built Faster Act. Key local rules:
- Up to 3 units permitted on most residential lots as-of-right
- Principal dwelling + 1 internal ARU + 1 detached ARU (garden suite)
- Minimum lot area requirements apply for detached ARUs
- Parking: 1 space required per ARU (in most zones)
- Owner-occupancy: London does not require owner-occupancy for ARUs
- Building permit required for all new ARUs and legalizations
- Rental licensing may be required — check with City