Second Residential Units
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​Second Residential Units

Second units (also known as accessory or basement apartments, secondary suites and in-law flats) are self-contained residential units with kitchen and bathroom facilities within dwellings or within accessory structures (for example, within a detached garage).
Examples of second units

Where are Second Residential Units Permitted?

  1. Second units are permitted in residential areas within single detached, semi-detached, and townhome/rowhouse dwellings or within a building accessory to these dwelling types (but not within both buildings on the same property).  Second units are not permitted within any other dwelling type (e.g. a duplex, double duplex, or multiple dwelling). To confirm whether a second unit is permitted, please contact the Planning Division by referring to the contact information provided below.
  2. In floodplain areas, second units are not permitted within basements. See map of areas regulated by the Essex Region Conservation Authority (ERCA). For more information regarding the location of floodplain areas please contact ERCA at 519-776-5209  or
  3. Outside of floodplain areas, basement units may be permitted where downspouts have been disconnected, a sump pump has been installed, and (where possible) a backflow prevention valve has been installed. Click the following links for more information regarding:

Land Use Policy and Zoning Regulation

Below is a summary of land use policies and zoning regulations applicable to second residential units. (Please see the Chapter 6 of the Official Plan and Section 5.99.80 of Zoning By-law 8600 for detailed requirements.)


Second units must be smaller than the primary dwelling unit. Specifically, the maximum gross floor area (GFA) of a second unit can be no larger than 40% of the GFA of the entire single detached, semi-detached, or townhome/rowhouse dwelling (i.e. including the proposed second unit) up to an absolute maximum GFA of 100 square metres or 1,076 square feet.

​GFA of Entire
Residential Dwelling
​Maximum GFA of
Second Unit

​100 m2 (1,076 sq. ft.)

​40 m2 (430 sq. ft.)
​140 m2 (1,500 sq. ft.) ​56 m2 (600 sq. ft.)
​186 m2 (2,000 sq. ft.) ​74 m2 (800 sq. ft.)
​232 m2 (2,500 sq. ft.) ​93 m2 (1,000 sq. ft.)
​279 m2 (3,000 sq. ft.) ​100 m2 (1,076 sq. ft.)
​325 m2 (3,500 sq. ft.) ​100 m2 (1,076 sq. ft.)


One additional parking space must be provided for a second residential unit, except for properties located within older, core areas of the city that have smaller lots sizes and are well served by transit. See map of the area where an additional parking space is not required.

Heritage Properties

Properties listed on the Municipal Heritage Register or located within a or located within a heritage area cannot alter any part of the exterior of the dwelling that is visible from the street. Please contact the Heritage Planner at 519-255-6543 ext. 6179 if you have questions about the Municipal Heritage Register. 

Accessory Buildings

Second residential units located within accessory buildings:

  • Must have access to a paved public street, municipal sanitary sewer, municipal storm water outlet, electrical, and water services;
  • Must have direct pedestrian access from the street or alley (i.e. tenants must not be required to access the second unit through the primary unit); 
  • Must have a municipal address clearly visible from the street; and
  • Cannot be severed from the property containing the primary dwelling unit.
  • The maximum height of accessory buildings in most residential areas is 5.5 metres (18 feet). The maximum height may be increased to 8 metres (26 feet) through minor variance to permit a second unit above a garage. See information about applying for minor variance.

For questions related to second unit policies and zoning regulations contact:

Planning & Building Services Department
Planning Division
2nd Floor, 350 City Hall Square West
Windsor, Ontario, Canada N9A 6S1
Phone: (519) 255-6543

Building Permit Required

Each building can have a different set of circumstances that may affect the design of a second residential unit. Part 9 of the Ontario Building Code (OBC) applies to the design and creation of second units. Specifically, the sections of the OBC listed below must be addressed while designing a proposed second unit:

  • 9.4 – Structural Requirements
  • 9.5 – Design of Area, Spaces and Doorways
  • 9.7 – Windows, Doors and Skylights
  • 9.8 – Stairs Ramps, Handrails and Guards
  • 9.9 – Means of Egress
  • 9.10 – Fire Protection
  • 9.11 – Excavation
  • 9.31 – Plumbing Facilities
  • 9.32 – Ventilation
  • 9.33 – Heating and Air-Conditioning

When a Second Unit is created, the OBC requires a distinct physical separation between the individual dwelling units. This can be demonstrated by submitting plans to the City’s Building Division that confirm existing conditions meet the OBC requirements or that proposed new construction/renovation will meet the OBC requirements. A qualified designer will be able to assess the current construction of a building and prepare the appropriate drawings to accompany a building permit application. 

For more information on secondary units, please see the following link: Add a Second Unit to Your House.

A building permit is required for any second unit.  Units created before November 20, 2018 will not be grandfathered.  See information about submitting a building permit application.

To submit a building permit application, contact:

Planning & Building Services Department
Building Division
2nd Floor, 350 City Hall Square West
Windsor, Ontario, Canada N9A 6S1
Phone: (519) 255-6267

Second Residential Units and Shared Rental Housing

The addition of a second unit does not change the total number of lodgers (i.e. persons that may be provided with lodging for hire) within the entire residential dwelling (i.e. both primary and second units).  Currently, more than three lodgers requires a property to be zoned and licensed for use as a lodging house. 

Proposed Changes from Second Units to Additional Dwelling Units (ADUs)

Municipalities in Ontario were mandated to have policies permitting additional residential units (previously referred to as second units) in 2011 under Bill 140, the Strong Communities through Affordable Housing Act. In the summer of 2019, the Province of Ontario passed Bill 108, More Homes, More Choice Act, which amended parts of the Planning Act to require municipalities to establish Official Plan policies and zoning that authorize the use of additional residential units. For more information, visit More Homes, More Choice: Ontario’s Housing Supply Action Plan.

The City of Windsor has initiated amendments to the official plans and zoning by-laws to permit an additional residential dwelling unit within a detached house, semi-detached house, or rowhouse, and within an accessory structure on the same property. This has the effect of potentially allowing a total of three dwelling units on the same property, subject to applicable provisions in the Ontario Fire Code, Building Code and municipal by-laws.

Proposed amendments were presented to the Development and Heritage Standing Committee at the March 9, 2020 meeting and were approved by City Council at the May 4, 2020 meeting. This website will be updated once the by-laws go into effect in June 2020. See summary of the proposed amendments.

For general inquiries, please contact 311.