ONTARIO HOUSING GUIDE
What Is an Additional Residential Unit (ARU) in Ontario?
A practical 2026 guide to basement apartments, secondary units, garden suites, zoning, building permits and Ontario’s Additional Residential Unit framework.
The Short Answer
An Additional Residential Unit, usually shortened to ARU, is an additional self-contained residential unit located in a house or, in some circumstances, in an accessory building on the same property.
Common examples include a basement apartment, secondary suite, in-law suite, unit above a garage, coach house or garden suite.
Ontario’s planning framework generally permits up to three residential units on qualifying urban residential land, but that does not mean every lot can automatically accommodate every type of unit. Building permits, municipal zoning standards, servicing, access, parking and other property-specific requirements still matter.
What Counts as an ARU?
An ARU is generally a separate residential unit with the facilities needed for independent living, such as sleeping space, a kitchen and bathroom facilities.
You may hear municipalities, builders and homeowners use several different terms for essentially similar forms of housing:
- Additional Residential Unit
- ARU
- Accessory Dwelling Unit
- ADU
- Secondary suite
- Second unit
- Basement apartment
- Accessory apartment
- Garden suite
- Coach house
- Laneway suite
- In-law suite
The terminology used by your municipality may differ, so always check the local zoning bylaw and municipal guidance that applies to the property.
How Many Units Can You Have?
Ontario’s Planning Act generally prevents municipal zoning bylaws from prohibiting certain three-unit configurations on qualifying urban residential land.
Common configurations can include:
- Three residential units in the main house, with no residential unit in an ancillary building; or
- Up to two residential units in the main house plus one residential unit in an ancillary building, such as a qualifying garden suite or converted accessory building.
This is a provincial land-use permission. It does not eliminate the need to comply with the Building Code, municipal zoning standards that remain applicable, servicing requirements or other approvals.
Does This Apply Everywhere in Ontario?
No. The province’s broad three-unit framework applies to qualifying urban residential land. Rural, agricultural, environmentally sensitive and otherwise specially regulated properties can require a different analysis.
For example, properties in prime agricultural areas are subject to separate provincial planning policies. Conservation authority regulation, septic capacity, private wells, environmental constraints and municipal zoning can also affect rural ARU proposals.
If a property is outside a conventional serviced urban neighbourhood, do not assume the same rules apply without checking.
Do You Still Need a Building Permit?
Yes, creating a new residential unit generally requires a building permit.
The building permit process is separate from the question of whether the residential use is permitted by zoning. Municipal building officials review the proposed construction for compliance with the Ontario Building Code and other applicable law.
Depending on the project, plans can need to address:
- Floor plans and room sizes
- Ceiling heights
- Windows and exits
- Fire separations
- Smoke and carbon monoxide alarms
- Heating and ventilation
- Plumbing
- Electrical work
- Structural changes
- Entrances and stairs
What About Parking?
Ontario’s ARU regulation limits certain municipal parking restrictions. The provincial framework generally provides for one parking space per additional residential unit, subject to exceptions, and tandem parking can be permitted.
If the local zoning bylaw requires no parking space for the primary residential unit, the provincial regulation also provides that a dedicated parking space is not required for the additional units.
Always confirm the current municipal parking standard because local rules can be less restrictive than the provincial maximum.
Can an ARU Be in a Detached Building?
Potentially. Ontario’s framework includes situations where one additional residential unit can be located in an ancillary building or structure on the same lot.
Depending on the municipality, this type of unit might be called a garden suite, coach house, laneway suite, accessory dwelling unit or detached ARU.
A detached unit can raise additional questions involving:
- Building setbacks
- Maximum height
- Lot coverage
- Distance from the main house
- Fire department access
- Water and sewer servicing
- Tree protection
- Conservation restrictions
- Parking and pathways
Does an Existing Basement Apartment Automatically Count as a Legal ARU?
No. A second kitchen, separate entrance or tenant in the basement does not by itself prove that the unit was legally established.
If an existing unit matters to a purchase or renovation, investigate items such as municipal records, permits, inspections, zoning and any required registration or licensing.
Before You Build an ARU
- Confirm the property’s zoning and whether the provincial ARU framework applies.
- Determine whether the proposed unit will be inside the house or in a detached structure.
- Review municipal setbacks, height, lot coverage and parking requirements.
- Confirm water, sewer or private servicing capacity.
- Check for conservation, heritage or environmental restrictions.
- Prepare the drawings required for a building permit.
- Obtain required permits before construction.
- Complete required municipal inspections.
- Confirm any registration or licensing requirements before occupancy.
FAQ
Additional Residential Unit Questions
How many residential units can I have on my property in Ontario?
Ontario’s Planning Act generally permits certain configurations of up to three residential units on qualifying urban residential land. The exact configuration and property-specific requirements still need to be verified.
Is a basement apartment an ARU?
It can be. A self-contained basement apartment can be an additional residential unit when it is legally established and complies with applicable planning, building and municipal requirements.
Can I build a garden suite?
Potentially. Ontario’s ARU framework allows an additional unit in an ancillary structure in certain circumstances, but municipal zoning, servicing, setbacks, access and building-permit requirements still apply.
Do I need a permit for an ARU?
Creating a new residential unit generally requires a building permit. Contact the municipal building department before starting work.
Official Ontario Resources
- Ontario Citizen’s Guide — Zoning Bylaws
- Planning Act
- O. Reg. 299/19 — Additional Residential Units
- Ontario Second Unit Guide
Last reviewed: September 6, 2026.
General information only: Ontario Home Intel provides general educational information and is not a substitute for legal, planning, building-code, engineering, architectural, tax, real estate or other professional advice. ARU permissions and requirements vary by property and municipality and can change. Confirm current requirements with the applicable municipality and qualified professionals before relying on this information.