Legal Basement Apartment Requirements in Ontario (2026 Complete Guide)
Adding a basement apartment to a home in London, St. Thomas, or Strathroy is one of the most popular ways Ontario landlords increase rental income. A well-done basement suite in London can generate $1,200–$1,600/month in additional rent. But doing it wrong — skipping permits, underbuilding the ceiling height, or missing a fire separation requirement — can result in fines up to $25,000, an order to stop renting immediately, and potential liability if something goes wrong with a tenant.
This guide walks through every requirement you need to meet in 2026 to create a legal, compliant secondary dwelling unit in Ontario.
What Is a "Second Unit" Under Ontario Law?
Ontario's Planning Act and the Ontario Building Code (OBC) use the term second dwelling unit or accessory dwelling unit (ADU) to describe a self-contained residential unit within an existing house. It must have its own kitchen, bathroom, and sleeping area.
Under Bill 23 (the More Homes Built Faster Act), municipalities across Ontario — including London, St. Thomas, and Strathroy — are now required to permit at least one additional residential unit in most residential zones as of right. You generally do not need a rezoning or minor variance, but you still need a building permit.
Step 1: Verify Your Zoning
Before spending a dollar on construction, confirm your property is in a zone that allows a second unit. In most cases it is, thanks to Bill 23 — but the process still starts at your local municipality:
- London, Ontario: Contact the City of London Building Division or check the Zoning By-law (ZBL-1) portal. Most R1 through R6 zones now permit second units as of right.
- St. Thomas: Contact the City of St. Thomas Planning Department.
- Strathroy-Caradoc: Contact the Municipality's Building Department directly.
Even where second units are permitted as of right, there may be lot coverage limits, setback requirements, or owner-occupancy rules that apply. Confirm before you build.
Step 2: Pull a Building Permit
A building permit is not optional. Constructing or converting a basement into a rental unit without one is illegal under the Ontario Building Code Act.
Typical permit costs (varies by municipality):
- London, Ontario: Approximately $600–$1,200 for a residential addition/conversion, depending on construction value.
- St. Thomas / Strathroy: Often similar, sometimes lower for smaller municipalities.
Submit plans to your local Building Department showing: floor plan, ceiling heights, window sizes, fire separations, and plumbing/mechanical layout. A Building Inspector will visit at key stages (framing, insulation, fire separation, final).
Step 3: Meet Ontario Building Code Minimums
This is where most illegal basement apartments fail. The 2026 OBC sets these non-negotiable standards for second units:
Minimum Ceiling Height
Habitable rooms (bedrooms, living areas) must have a minimum ceiling height of 1.95 metres (approximately 6 feet 5 inches). Hallways and bathrooms can be slightly lower at 1.8 m.
This measurement is from the finished floor to the finished ceiling. If your joists run through that space, you need to account for the finished drywall below them. Many older London homes have basement slabs that put the finished ceiling right at the limit — measure carefully before committing.
Egress Windows in Every Bedroom
Every bedroom in a basement apartment must have a window that meets the OBC egress requirements for emergency escape:
- Minimum unobstructed opening area: 0.35 m²
- Minimum opening height: 380 mm
- Minimum opening width: 380 mm
- Maximum sill height above finished floor: 1,000 mm (so a tenant can actually climb out)
Standard basement windows almost always fail this test. Plan on cutting larger window wells and installing larger windows. This is typically a $1,500–$3,000 per window cost but it is a life-safety requirement — non-negotiable.
Fire Separation Between Units
The basement unit must be fire-separated from the rest of the house with a 45-minute fire resistance rating. In practice this means:
- Ceiling/floor assembly: The floor above the basement unit (which is the main floor of the house) must achieve 45-minute fire resistance. Typically achieved with 5/8-inch Type X drywall on the basement ceiling, properly taped, mudded, and finished with no penetrations.
- Common walls: Any shared wall between units needs the same treatment.
- Doors between units: Any door connecting the basement unit to the upper unit must be a solid-core, self-closing door with proper fire-rated weatherstripping and a closing mechanism. Hollow-core interior doors do not qualify.
- Penetrations: Any pipe, wire, or duct that passes through the fire separation must be fire-stopped with approved caulking or fire-stop collars. This is one of the most commonly missed requirements on final inspection.
Smoke and Carbon Monoxide Alarms
The Ontario Fire Code requires:
- A smoke alarm on every storey of the dwelling, including within each sleeping area.
- A carbon monoxide (CO) alarm adjacent to every sleeping area in any unit that has a fuel-burning appliance or an attached garage.
- Interconnected smoke alarms are required — when one goes off, all should sound.
CO alarms are especially critical in basements, where furnaces and hot water heaters are often located.
Heating and Ventilation
The basement unit must have its own independent heat source capable of maintaining 21°C year-round. The landlord cannot legally charge a tenant for heat if the only heat source is the shared furnace. Options: separate electric baseboard heaters, a separate zone on the furnace with its own thermostat, or a ductless mini-split.
Proper ventilation (including a bathroom exhaust fan vented to the exterior) and kitchen ventilation are also required under OBC Section 9.32.
Landlord Insights
Get practical tips for Ontario landlords — delivered free.
Step 4: Separate Entrance
Ontario's OBC and most municipal zoning bylaws require the second unit to have its own exterior entrance separate from the main unit's entrance. This can be from the front, side, or rear of the home.
This requirement exists for fire safety (so each unit can exit independently without passing through the other unit) and privacy. An interior staircase between units is permitted only if it has a fire-rated door — but it cannot be the sole means of entry for the basement tenant.
Step 5: Electrical — ESA Certificate
Any electrical work in the basement unit requires:
- A licensed electrical contractor to do the work (or a homeowner permit, which has limitations).
- An inspection and certificate from the Electrical Safety Authority (ESA).
The ESA certificate is not part of the municipal building permit process — it runs in parallel. You will need proof of ESA approval before you can legally rent the unit.
Common electrical upgrades for basement apartments: updated panel with adequate capacity, GFCI outlets in bathroom and kitchen, arc-fault circuit interrupters (AFCIs) on bedroom circuits, and separate metering if you want to bill utilities separately.
Step 6: 2026 Building Code Updates to Know
The 2024/2026 OBC updates introduced two requirements particularly relevant to basement apartments:
- Radon protection: New construction and significant renovations in Ontario must include radon mitigation measures. For basement apartments, this means installing a sub-slab radon rough-in (a passive pipe system) during construction. Radon is a naturally occurring radioactive gas that accumulates in basements and is the second leading cause of lung cancer in Canada. Health Canada recommends action if levels exceed 200 Bq/m³.
- Thermal insulation upgrades: Basement walls that are opened for renovation now must meet current insulation minimums — typically R-20 for below-grade walls in London's climate zone.
The $25,000 Fine Risk
Operating an illegal basement apartment in Ontario exposes you to:
- A Provincial Offences Act fine up to $25,000 per day of non-compliance.
- An order from the municipality to immediately vacate the unit.
- Liability if a tenant is injured or killed in an unpermitted unit — your insurance may not cover you.
- An RTA headache: under the Residential Tenancies Act, a tenant in an illegal unit can apply to the LTB for rent abatement or termination, and courts have ordered substantial refunds.
Municipalities across Southwestern Ontario have stepped up enforcement in recent years. London's bylaw enforcement team responds to anonymous complaints — including from tenants who later want out of a lease.
How Long Does It Take?
A realistic timeline for a permit-to-occupancy basement apartment conversion in London or St. Thomas:
- Permit application review: 2–6 weeks
- Construction (ceiling, egress windows, fire separation, electrical, plumbing): 4–8 weeks depending on contractor availability
- ESA inspection and certificate: 1–2 weeks after electrical work is done
- Municipal final inspection: 1–3 weeks after construction is complete
Budget 3–5 months from permit application to legal occupancy, and a total construction cost of $25,000–$60,000 depending on the scope of work and existing conditions.
Frequently Asked Questions
Do I need a separate hydro meter for a basement apartment in Ontario? No — Ontario law does not require a separate meter for a second unit. However, if you want to charge the tenant separately for utilities, you need either a separate meter or a written allocation formula in the lease. Without one, utilities are the landlord's responsibility. Many London landlords simply include utilities in the rent for basement units to avoid the complexity.
Can I convert my basement apartment without telling my mortgage lender? Technically your mortgage terms may require you to notify your lender of changes to the property. More practically, your homeowner's insurance almost certainly needs to be updated to reflect the rental use — failure to disclose can void your coverage in the event of a claim. Contact your insurer before tenants move in.
What happens if I inherited a house with an existing illegal basement apartment? You still own the liability. You have two options: bring the unit up to code (pull permits retroactively, which is allowed in most Ontario municipalities) or stop renting it. "It was like this when I bought it" is not a legal defence under the OBC or Ontario Fire Code.
Can my basement tenant use the same entrance as me? Only if it also qualifies as a proper means of egress from the basement unit. In practice, most Building Departments require a separate exterior exit for the basement unit for fire safety reasons. An interior shared door is allowed as a secondary door but rarely as the only entrance.
Do these rules apply to above-ground secondary suites (like a garage apartment or laneway house)? The same OBC requirements for fire separation, smoke/CO alarms, and egress apply to any secondary dwelling unit. Ceiling height and below-grade-specific rules (radon, insulation) may differ for above-ground structures. Zoning treatment of laneway homes and garden suites also varies significantly by municipality.
Getting It Right the First Time
A legal basement apartment is one of the best investments you can make as a landlord — but only if it's done properly. Done right, it adds real appraised value to your property, generates steady rental income, and gives you a liability-free tenancy.
If you're unsure where to start — permits, contractors, or whether your property qualifies — the team at Prospera Properties works with London, St. Thomas, and Strathroy landlords regularly and can point you in the right direction. We've seen the full range of basement conversions, legal and otherwise, and can help you avoid the costly mistakes.
For more on managing your rental income once the unit is ready, see our guides on first-time landlord tips in London, Ontario, landlord maintenance responsibilities in Ontario, and rental property tax deductions in Ontario — a properly permitted basement unit opens up deductions that an illegal one does not.
Ready to make the most of your property? Contact Prospera Properties to talk through your options.