Additional Dwelling Units/Second Residential Units

Additional Dwelling Units (also known as an ADUs, Second Units, accessory or basement apartments, secondary suites, carriage houses, laneway 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).

Note to reader: Please see the proposed Official Plan Amendment 172.

Examples of additional dwelling or second residential units 

The informative video below provides an overview of Additional Dwelling Units as the permitting process to legally establish these units:

Additional Dwelling Units video

Where are Additional Dwelling Units (ADUs) Permitted?

  1. ADUs are permitted in residential areas within single detached, semi-detached, and townhome/rowhouse dwellings and/or within a building accessory to these dwelling types. ADUs are not permitted within any other dwelling type (e.g. a duplex, double duplex or multiple dwelling). To confirm whether an ADU is permitted, please contact the Planning Division by referring to the contact information provided below.
  2. In floodplain areas, ADUs 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 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 Additional Dwelling Units. (Please see the Chapter 6 of the Official Plan and Section 5.99.80 of Zoning By-law 8600 for detailed requirements.)


Additional Dwelling Units are intended to be smaller than the primary dwelling unit. The minimum required gross floor area (GFA) is 40 square metres or 430.6 square feet, up to a maximum GFA of 100 square metres or 1,076 square feet.


One additional parking space must be provided for an ADU, 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. If a second ADU is added to a lot, no additional parking space is required for that unit.

Heritage Properties

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

Accessory Buildings

Additional Dwelling Units located within accessory buildings (detached ADUs):

  • 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.
  • Must have minimum side and rear setbacks of 1.2 metres. If upgrading or adding to an existing garage to include an ADU, the 1.2 metre setback only applies to the portion of the building that is being altered.
  • For a detached ADU, must adhere to maximum height restrictions dependent on the roof slope:
    • For roofs with a slope of 20 degrees (approximately 4/12 roof pitch) or more the maximum height is 8 metres (26 feet).
    • For roofs with a slope of less than 20 degrees, the maximum height is 6 metres. The maximum height for low slope or flat roofs may be increased to 8 metres (26 feet) through minor variance. See information about applying for minor variance.
  • For a detached ADU, must not have a maximum height exceeding the building height of the main building on the lot. For example, a two-storey ADU is not permitted on a lot where the main building is only a single storey.

Please see Section 5.99.80 of Zoning By-law 8600 for detailed requirements. For questions related to ADU 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 an Additional Dwelling Unit. Part 9 of the Ontario Building Code (OBC) applies to the design and creation of ADUs. Specifically, the sections of the OBC listed below must be addressed while designing a proposed ADU: 

  • 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 ADUs within the main building (for example, basement or attic suites), please see the followingAdd a Second Unit to Your House.

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

For questions relating to submitting 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

Additional Dwelling Units and Shared Rental Housing

The addition of an ADU 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 additional units). Currently, more than three lodgers requires a property to be zoned and licensed for use as a lodging house. 

Tiny Homes

Please note that tiny homes is not a defined term in the Ontario Building Code or the City's Zoning By-law. If you are considering building a small home in your backyard, this would be considered a detached Additional Dwelling Unit. The Zoning By-law and Ontario Building Code regulate minimum dwelling unit size. Further, the Zoning By-law does not permit a motor home, recreational vehicle or trailer to be used as a dwelling unit. Please consult a qualified designer for specifics related to your project.

Changes from Second Units to Additional Dwelling Units (ADUs)

Municipalities in Ontario were mandated to have policies permitting 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, the 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.

City of Windsor amendments to implement ADUs 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. The by-laws that implemented changes to ADU regulations went into effect on June 26, 2020.

For general inquiries, please contact 311.