What Is the Maximum Garden Suite Size in Ontario?

If you are planning to build a garden suite in Ontario, one of the most common questions homeowners ask is:

How big can a garden suite be on my property?

There is no single province-wide maximum size. The permissible garden suite area depends on:

• Municipal zoning by-laws

• Maximum lot coverage percentage

• Required setbacks

• Height restrictions

• Ontario Building Code requirements

Most municipalities allow garden suites between 40 m² and 80 m² (430 – 860 sq ft), but the final permitted size depends on your specific property.

What Is a Garden Suite?

A garden suite (also called a detached Additional Residential Unit or ARU) is a self-contained dwelling located in the backyard of a property with an existing house.

Across Ontario, residential properties may permit:

• One primary dwelling

• One internal secondary unit (such as a basement apartment)

• One detached garden suite

However, municipalities regulate size, height, and placement.

How to Calculate the Permissible Garden Suite Area

1. Check Your Lot Coverage

Lot coverage limits how much of your property can be covered by buildings.

Example:

• Lot size: 5,000 sq ft

• Maximum lot coverage: 40%

5,000 × 40% = 2,000 sq ft total permitted building footprint

Now subtract:

• Existing house footprint

• Garage

• Covered porches

• Sheds or accessory structures

The remaining footprint may be available for your garden suite.

2. Confirm the Municipal Size Cap

Even if lot coverage allows a large footprint, your municipality may cap garden suites at:

• 60 m² (645 sq ft), or

• 80 m² (861 sq ft)

You must comply with both limits. The smaller value applies.

3. Review Setback Requirements

Setbacks reduce your buildable area.

Typical Ontario garden suite setbacks include:

• Rear yard: 1.2 m – 3.0 m

• Side yard: 1.2 m – 1.8 m

• Minimum separation from the main dwelling

On narrow or irregular lots, setbacks can significantly reduce the permitted footprint.

4. Check Height Restrictions

Most municipalities allow:

• One storey

• Maximum building height between 4.5 m and 6 m

Height restrictions affect roof design and interior layout.

5. Ensure Ontario Building Code Compliance

All garden suites must comply with the Ontario Building Code (OBC), including:

• Minimum room sizes

• Ceiling height requirements

• Egress window standards

• Fire-resistance ratings

• HVAC and ventilation compliance

• Plumbing servicing

Even if zoning permits a certain size, building code requirements may impact final design efficiency.

Typical Garden Suite Size Ranges in Ontario

Small Urban Lot:
400 – 600 sq ft

Standard Suburban Lot:
600 – 800 sq ft

Large Lot:
Up to 861 sq ft (subject to zoning limits)

Oversized backyard dwellings may require a minor variance application.

Common Mistakes When Determining Garden Suite Size

• Ignoring lot coverage calculations

• Forgetting to subtract existing structures

• Overlooking setback requirements

• Designing before confirming zoning compliance

• Assuming provincial rules override municipal by-laws

Proper zoning review before design prevents costly permit delays.

Final Thoughts on Garden Suite Permits in Ontario

The maximum garden suite size in Ontario depends entirely on municipal zoning rules and your property’s physical characteristics.

Before starting architectural drawings:

• Confirm zoning permissions

• Calculate available lot coverage

• Review setbacks

• Check height limits

• Verify building code compliance

Proper planning ensures a smoother garden suite permit approval process.

Disclaimer

This article is provided for general informational purposes only. Garden suite regulations vary by municipality and may change. Property-specific factors such as zoning classification, lot configuration, easements, conservation authority regulations, servicing limitations, and local by-law amendments may affect permissible building area. Always verify requirements with your local municipality and consult qualified professionals before proceeding with design or construction.