What makes a basement apartment legal in London, Ontario?

A London basement apartment must comply with applicable zoning, receive a building permit before construction, meet Ontario Building Code and Fire Code requirements, pass the required inspections, and maintain a City residential rental unit licence when rented.

Do I need a building permit to legalize an existing basement apartment?

Yes. The City of London lists an additional residential unit, basement apartment, basement living-space renovation, below-grade entrance, and excavation to increase headroom among work that requires a building permit.

What drawings are required for a London basement ARU permit?

The City requires existing floor plans for all floors and proposed plans showing walls, fire separations, windows and doors, room uses, plumbing, ceiling heights, egress and exits. Elevations may also be required when windows or doors change.

Can I legalize a basement with a low ceiling?

The applicable Building Code path depends on the house and project. Ontario guidance identifies 1.95 metres over the required floor area and exit route as a common second-unit benchmark. A qualified designer or engineer should confirm whether selective work, benching, underpinning or another compliant design is feasible.

2026 London homeowner guide

Legal Basement Requirements in London, Ontario

A legal basement is not defined by one window measurement or one layer of drywall. It is the result of a permit-first process that coordinates zoning, drawings, structure, fire safety, egress, mechanical work, inspections and rental licensing.

Last reviewed: July 21, 2026. Based on the City of London's ARU and building-permit guidance and Ontario's second-unit guidance. Requirements can change and every property is different.

The short answer

To legalize a basement apartment in London, confirm zoning, obtain an approved building permit before construction, complete the OBC and Fire Code work shown on the permit drawings, pass inspections, close the permit, and obtain the required residential rental unit licence before operating it as a rental.

The eight-part legalization path

1. Property and zoning

Confirm the property type, zone and project fit. London currently permits up to four total dwelling units where the underlying residential use is permitted, subject to current ARU restrictions and property-specific review.

2. Permit strategy

A basement apartment and work such as a new below-grade entrance, increased headroom, interior alterations, HVAC changes and enlarged openings require permit review. Do not start demolition or structural work first.

3. Complete drawings

Plan sets should document existing floors and proposed walls, doors, windows, rooms, plumbing, ceiling heights, exits, egress and fire separations. London says single-line drawings are not acceptable.

4. Ceiling and structure

Ontario guidance uses 1.95 m over the required basement second-unit floor area and route to the exit as a common benchmark. Low areas may require redesign, selective excavation, bench footing or underpinning.

5. Exits and egress

The correct path depends on unit layout and the applicable Code route. Measure the clear openable window—not the frame—with the Ontario egress calculator, then have the designer confirm the full exit strategy.

6. Fire and life safety

Fire separations must work as continuous assemblies around ducts, services, doors and common spaces. Smoke and carbon-monoxide alarms, closures and penetrations must match the approved design. Read the fire separation guide.

7. Mechanical and electrical

Heating, ventilation, plumbing and electrical changes must be coordinated with the architectural plans and completed under the required permits and inspections. London may require a qualified BCIN HVAC designer.

8. Inspections and licensing

Book inspections at the stages listed on the permit, correct deficiencies, close the building permit and maintain the City's residential rental unit licence if the unit is rented.

Interactive basement legalization readiness check

Check only items you can prove with a document, measurement or professional review. This is planning triage—not permit approval.

0 of 8 proof items ready

Zoning checked: property type, zone and current ARU limits reviewed. Existing conditions measured: all floors, rooms, windows, doors, beams, ducts and ceiling heights documented. Designer identified: homeowner-design route or qualified BCIN designer/architect/engineer confirmed. Exit and egress strategy: clear opening measurements and travel path reviewed. Fire separation strategy: ceilings, walls, doors, ducts and penetrations mapped. Mechanical scope: HVAC, plumbing, electrical and service capacity reviewed. Permit issued: work has not started ahead of permit issuance. Close-out path: inspections, deficiencies, permit closure and rental licensing budgeted.

What to bring to a first professional review

Property address, survey or site plan, and any old permit records.
Photos and measured sketches of every floor—not only the basement.
Clear ceiling heights under finished surfaces, beams and ducts.
Window clear-opening dimensions, well depth and sill heights.
Known plumbing, HVAC, electrical-panel and foundation constraints.
Your intended use: family unit, long-term rental, existing occupied unit or enforcement response.

Get a plain-English readiness review

Tell us the main obstacle and we will route your request to the most relevant next-step guide or vetted London professional. No public phone CTA and no obligation.

Primary sources

City of London: Additional Residential Units (last modified June 5, 2026)
City of London: Before you apply for a building permit
Ontario: Add a second unit in your house
Ontario Building Code, O. Reg. 332/12

Educational information only. The approved permit drawings and the authorities having jurisdiction determine the requirements for a specific property.