Accessory Structure

Sometimes referred to as a “secondary structure,”  accessory structures are incidental and subordinate to the use of the main dwelling unit that is on the same lot. The accessory structure should also be physically separate from the main dwelling on the lot in which both buildings are located. 


An additional dwelling unit (ADU) is a secondary living area to the main dwelling unit that includes private kitchen and bathroom facilities. It can be within or attached to the main dwelling unit or completely detached from it. While these concepts might vary across jurisdiction, generally ADUs within or attached to a main dwelling unit can also be found under names such as secondary suites, basement apartments, subsidiary dwelling units, secondary dwelling units – attached, etc. On the other hand, detached ADUs can also be found under variations of garden suites, laneway suites, garage suites, carriage houses, coach houses, in-law flats, etc. Generally, most municipalities restrict the size of an ADU so that it is smaller than the main dwelling unit and disallow the lot from being legally severed or subdivided, as they are considered additional or accessory to the main unit.

Unless otherwise stated, the use of the term “ADU”  in and throughout this website refers to detached ADUs.  

ADU Suitability

ADU Suitability refers to our determination of the suitability of a lot to accommodate an ADU. We calculated the level of suitability according to relevant zoning bylaw requirements and other external regulatory provisions affecting ADU development in a municipality. Although these requirements are unique for all municipalities, ADU suitability is generally determined by considering the following factors: zoning, building type of the MDU, the presence of an MDU on the lot, the presence and/or number of accessory structures on the lot (e.g., a garage), lot size, main dwelling unit size, lot coverage restrictions (for all and accessory buildings), ADU buildable area size, and different environmental factors (e.g., proximity to floodplains or conservation areas). 

ADU Suitability will surface as one of four possible outcomes based on our calculations and analysis of the regulatory provisions:

  1. Not Suitable: This outcome will color the lot in red and means that it is NOT possible to build an ADU (either internal or external) on the lot.
    • The factors that may deem a lot  “Not Suitable” vary by municipality but some examples include:
      • Vacant Lots:  ADU development is often not permitted on lots that do not have a main dwelling unit. 
      • Main Dwelling Unit Type: The main dwelling unit is not the allowable building type (e.g., the main dwelling unit is a legal duplex, and the municipality does not allow ADUs in that building type).
      • Main Dwelling Unit Size: Main dwelling unit size does not meet, or exceeds, the regulated size requirements.
      • Zoning: Lot is not located in a zone that permits either the necessary building type for the main dwelling unit or does not permit the development of ADUs.
      • Lot Size: Lot size does not meet regulated size requirements.
    • The factors that cause lots to be deemed “not suitable”  normally require significant effort to change (e.g., applying to rezone a lot) and it is reasonably assumed that these factors would not be easily changed at an individual level.
  1. Suitable for Internal / Attached ADUs: This outcome will colour the lot in yellow, and means that it is NOT possible to build a detached ADU. However, building an attached or internal ADU is possible.
    • The factors that may deem a lot  “Suitable for Internal/Attached”  vary by municipality but some examples include:
      • Lot Coverage Violations: GIS calculations determined that the addition of a minimum-sized detached ADU would make the lot become in violation of total lot coverage maximums (i.e. all buildings on a lot) or accessory (i.e. only the accessory structures on a lot).
      • Existing Accessory Structures: Some municipalities allow for a limited number of accessory structures, and the addition of a detached ADU would make the lot in violation of the accessory structure maximum. 
      • Type/Number of Existing ADUs: Some municipalities limit the type (internal/attached or detached) and number of ADUs allowed on a lot. The addition of a detached ADU would make the lot in violation of the ADU number maximum. 
      • Lot Dimension Failures: One or more lot dimensions (e.g., frontage, width, depth, etc.) does not meet, or exceeds, the regulated size requirement.  
      • Minimum/Maximum Permitted ADU Size: The potential buildable area calculated is in violation of either the minimum or maximum detached ADU size requirement. 
    • The factors that cause a lot to be deemed “Suitable for Internal/Attached ADUs”  normally relate to size and coverage restrictions. Although the lot may not have enough space to build a detached ADU, an internal/attached ADU may be considered as base requirements (zoning, MDU building type, etc.) on the lot have been met. 
  1. Potentially Suitable: This outcome will color the lot in light green, and means that there are additional restrictions/requirements that need to be met before the lot is permitted to have a detached ADU.
    • The factors that may deem a lot “Potentially Suitable”  vary by municipality, some examples include:
      • Flood or Conservation Area Requirements: When a lot is located in or within the buffer of a flood plain or regulated conservation area, a site evaluation and permit is typically required before any development on the lot can take place.  
      • Detached Garage Considerations: ADUs built on detached garages are often subject to different setback and size requirements compared to ADUs built in a standalone detached structure, which cannot be accounted for on a 2-D mapping level. 
    • The factors that cause a lot to be deemed “Potentially Suitable”  can be seen as additional steps/considerations in the ADU planning and development process. 
  1. Suitable for Detached: This outcome will color the lot in dark green and means that the lot meets the requirements and a detached ADU can be built. 

Attached ADU

A type of ADU that shares at least one common wall with the main dwelling unit. 

Buildable Area

This space is calculated by configuring the appropriate setback from property lines and the main dwelling unit. It does not account for secondary structures (such as detached garages, pools, and sheds), and there may be some overlap between the ADU buildable area and these structures. It also does not take into account lot coverage restrictions but simply provides the area where a detached ADU can be built.

Conservation Authority Area

Unique to Ontario, Conservation Authorities are local watershed management agencies that deliver services and programs to protect and manage impacts on water and other natural resources in partnership with all levels of government, landowners, and many other organizations.

Detached ADU

A type of ADU that is located in a separate structure that is on the same lot and that does not share any common walls with the main dwelling unit. 

Dissemination Area

The smallest standard geographic measure in Canada, dissemination areas (DAs) are drawn to have a population between 400 and 700 people.

Environmental Protection Zone 

A zone within a municipality where the primary uses for lots within this area are aimed toward environmental protection such as ecological management, conservation, natural restoration, etc. Given these primary uses, ADU development is typically prohibited, or only allowed under certain conditions, within Environmental Protection zones. 

Existing Secondary Building

This details the area of any existing structure on the lot that is detached from the Main Dwelling Building. This information can be found in the sidebar of the tool under “Lot Information.” 


An area of land that is prone to flooding. Municipal bylaws and conservation authority regulations often restrict ADU construction in these areas or allow it with special considerations and permits. 

Gross Floor Area (GFA)

The sum of the total area of each floor level of a building, above and below the ground, measured from the exterior of the main wall of each floor level.

Heritage Conservation Districts/Zones

A district or area within a municipality that has a unique historical value or aesthetic character. Construction or changes within these districts might be prohibited or allowed with special permits and requirements. For the purposes of our tool, we do not flag properties falling within heritage districts. However, certain heritage districts have been flagged (e.g., Kitchener) under direct request from the municipality. 

Hypothetical Detached Regulation (HDR)

The Hypothetical Detached ADU Regulation (HDR) is a set of hypothetical bylaws for detached ADUs. Our team designed the HDR in consultation with ADU advocates from municipal planning services and the planning field to model the potential for detached ADUs in cities without existing regulations in Quebec.

Internal ADU

A type of ADU wherein the additional unit is located within the existing main dwelling unit (e.g., in the basement).

Lot Coverage

The percentage of the total lot area covered by all buildings and structures above finished grade. Please refer to the municipality’s zoning bylaws to know what is and is not included in this calculation (e.g., swimming pools, unenclosed porches, patios, decks, etc.). 

Lot Depth

The mean horizontal distance between the front and rear lot lines, measured within the lot boundaries.

Lot Frontage

The horizontal distance between side lot lines, or the projection of the side lot lines, measured along a straight line drawn perpendicular to the lot centreline at the required minimum front yard setback.

Lot Placement

The location of the lot in relation to its physical surroundings such as roads, lanes, roundabouts, etc. 

Lot Width

The maximum horizontal distance between parallel side lot lines. Non-parallel side lot lines may have unique lot width calculations that vary across municipalities. Please refer to the municipality’s zoning bylaws or contact the municipality for more details about lot width calculations. 

Main Dwelling Unit (MDU)

The principal building on a lot that is used for dwelling/residential purposes. Examples include, but are not limited to, a single family detached house, a semi-detached house, or a town/row house.

Maximum ADU Size

This represents the maximum possible footprint of a one-storey detached ADU based on multiple intersecting factors of the local municipal bylaws, including maximum allowable ADU size, buildable area calculations (i.e. total and accessory lot coverage restrictions), and land dimensions. This information can be found in the sidebar of the tool under “Lot Information” .

Maximum Lot Coverage

This refers to the maximum allowable lot coverage for a zone or subzone provided by the municipality. For example, a maximum lot coverage of 40% in zone R1 for municipality x implies that the total lot area for the main dwelling and all accessory buildings cannot exceed 40% of the total lot area. 

Mini-Home (maison préfabriquée mobile)

A prefabricated or “factory-built”  moveable dwelling containing 1 dwelling unit designed to be transported on its own chassis or undercarriage, connected to utilities and designed for year-round living. 

Minimum ADU Size

This represents the minimum footprint or gross floor area (GFA) of a one-storey detached ADU. It is not permitted to build ADUs with a GFA that is less than the regulated Minimum ADU Size. This value is usually determined by the municipality or by the building code to ensure safe and proper living conditions for the ADU occupants. This information can be found in the sidebar of the tool under “Lot Information.” 

National Minimum Regulation (NMR)

The National Minimum Regulation (NMR) applies a lenient set of zoning bylaw requirements for building detached ADUs on residential lots. Our team designed the NMR to conduct a baseline analysis in all cities to understand the maximum potential for building detached ADUs under a permissive policy scheme.

Railway Right-of-Way

A strip of land that is granted, through an easement or other mechanism, for transportation purposes. A right-of-way is reserved for the purposes of maintenance or expansion of existing services with the right-of-way. 


This is the minimum distance from a property line that is required for the erection of a new building. 

E.g., If the side, rear, and main dwelling unit setback for an ADU is 1.2 m, the ADU must have at least a 1.2 m distance between the ADU walls and side lot line, the rear lot line, and the main dwelling walls.

Through Lot

A lot other than a corner lot with frontage on more than one street other than an alley. In other words, a lot that is bound by streets on its front and rear. Through lots with frontage on two streets may be referred to as double frontage lots.



Yard Coverage

The percentage of the total yard area covered by all buildings and structures above finished grade. The boundaries for rear, side, and front yards may vary across municipalities. Please refer to the municipality’s zoning bylaws or contact the municipality for more information

Zoning (or Zoning/Planning District)

The act of categorizing the land by a governing entity (usually a municipality) into zones, where each zone has a unique set of rules and regulations that govern land use, development, and the types of permitted structures. For the purposes of our tool we focus on residential zones within municipalities.