Additional Residential Units (ARUs) in Ontario: Zoning, Building Code and Permits

Bill 23 and Additional Residential Units in Ontario

Bill 23 introduced important planning changes allowing more residential units on many urban residential lots in Ontario. In general, up to three residential dwelling units may be permitted on a parcel of urban residential land, depending on municipal zoning, servicing, lot conditions and the location of the units.

It is important to understand that Bill 23 is primarily a planning and zoning change. It does not exempt a project from the Ontario Building Code, Ontario Fire Code, municipal registration requirements, zoning setbacks, parking requirements, conservation authority approvals or servicing limitations.

Ontario Building Code 2024: House vs. Small Building

Under the Ontario Building Code, a house generally means a detached house, semi-detached house or row house containing not more than two dwelling units. This distinction is critical for ARU projects.

Architectural Requirements for Secondary Suites and ARUs

Fire Separation

Fire separations are required between dwelling units and between dwelling units and shared spaces. The required fire-resistance rating depends on the building classification, existing construction, age of the house, and applicable provisions of the Ontario Building Code.

Exits and Egress

Each dwelling unit must have safe exit access. Basement units require careful review of stairs, exterior doors, egress windows, headroom and travel distance. Municipal zoning may also affect the location of below-grade entrances.

Ceiling Height and Layout

Existing basement conditions must be reviewed carefully. Ceiling height, beams, ducts and bulkheads can impact whether a basement is suitable for a legal secondary suite.

Sound Separation

Sound separation between units is recommended and may include insulation, resilient channels and gypsum board assemblies.

Mechanical, Plumbing and Electrical Requirements

Heating Systems

Each dwelling unit must have adequate heating. This may include shared systems with zoning controls, separate HVAC systems or heat pumps depending on the design.

Ventilation

Proper ventilation is critical. Kitchens and bathrooms must be exhausted, and in many cases HRV or ERV systems are required to maintain indoor air quality.

Duct Penetrations and Fire Protection

Where ducts pass through fire separations, fire dampers or other fire protection measures may be required depending on the design and applicable code provisions.

Plumbing

Additional kitchens and bathrooms increase demand on plumbing systems. Drainage, water supply, sump pumps and backwater valves should be reviewed.

Electrical

Electrical systems must comply with ESA requirements. Panel capacity, smoke alarms, carbon monoxide alarms and potential separate metering should be reviewed.

Municipal Zoning Requirements for ARUs in Ontario

Each municipality has its own zoning requirements, terminology and approval process. The following provides a general overview.

For a detailed comparison of parking-space dimensions, accessible parking requirements, garage clearances, drive aisle widths, tandem parking provisions, and municipality-specific design standards across Toronto and the GTA, please refer to our article Parking Space Dimensions in Toronto and the GTA: Zoning Standards and Design Guide.

Design Approach Based on the Age of the House

Why Professional Design Is Important

A legal ARU project requires coordination between zoning, architecture, structural engineering, mechanical design and permit approvals. Proper design reduces delays and ensures compliance with Ontario regulations.

Additional Residential Unit Design Services

Deltoid Consulting provides architectural and engineering services for ARUs, secondary suites and residential conversions across Ontario, including zoning review, permit drawings and construction support.