ADUs in Ontario. The law calls them additional residential units.

Key takeaways

Updated October 15, 2026.

What an ADU is in Ontario

ADU is the term most people search for, borrowed from North American planning. Ontario's Planning Act and O. Reg. 299/19 use additional residential unit. Ontario's laneway house guide says these units are often called second or basement suites, accessory dwelling units, granny flats, coach houses, tiny homes or laneway suites. Your municipal by-law decides which forms count where you live.

The three kinds of ADU

• Inside the house: a basement or upper-floor suite.
• Attached: an addition with its own entrance.
• Detached: a garden suite in the back yard, or a laneway suite on a public lane. This is what Modular X builds.

What Ontario law allows

On a parcel of urban residential land with a detached, semi-detached or row house, Planning Act s. 35.1(1) stops zoning from prohibiting up to two additional units in the house, or one in the house and one in an ancillary building such as a garden suite. O. Reg. 299/19 says any person can live in an additional residential unit, whether or not they are related to the owner. Municipal zoning still sets height, setbacks, floor area and parking.

How a backyard ADU gets built

1. Lot feasibility: zoning, servicing, fire access and trees
2. Design to the by-law and the Ontario Building Code
3. Building permit
4. Foundation and services, site-built or modular
5. Inspections through to occupancy

See garden suites, laneway suites in Toronto and additional residential units in Ontario.

What an ADU costs

It depends on the lot: foundation, servicing distance, permits, fees and the build method. See garden suite cost in Ontario. We price your lot after feasibility.

Choose an ADU type before choosing a floor plan

An ADU can sit inside the main house, within an attached addition or in a separate backyard building. A basement suite uses existing building space but needs review of exits, fire separation, ventilation and other Code requirements. An attached unit changes the main building. A garden or laneway suite needs its own site position, foundation and service route.

These are different ways to add a self-contained home, not interchangeable permit applications. Count every existing residential unit before proposing another. The household arrangement can help guide accessibility and privacy choices, but it does not remove the need for a compliant kitchen, bathroom, sleeping space and safe access.

What up to three units as-of-right actually means

Planning Act sections 16(3) and 35.1 protect specified additional-unit arrangements on qualifying urban residential land with municipal water and sewage services. The configurations include three units in the main house, or two in the house and one in an ancillary building. The protection is not permission for three detached backyard buildings.

O. Reg. 299/19 addresses additional-unit standards including parking and occupancy. Read it alongside the Planning Act and the local by-law rather than treating any one source as the complete approval. Height, setbacks and other site requirements must still be checked for the proposed building. A rural or privately serviced property should not be assumed to qualify under the same provisions.

Toronto and Mississauga use different permit files

Toronto distinguishes a laneway suite on a qualifying public-lane lot from a garden suite. That distinction affects the design and access review before a catalogue plan is selected. The Toronto laneway housing guide explains the municipal terminology and process.

Mississauga organizes backyard units under its ARU program and provides its own garden-suite drawings and application requirements. Use the Mississauga ARU source for that property rather than importing Toronto dimensions or forms. In either city, a pre-approved design still needs the site-specific drawings and confirmations required for the lot.

Prepare the permit as a coordinated package

Begin with the property address, survey and a record of existing units. Establish the zone, proposed footprint, setbacks, lot coverage and a workable fire-access route. Check trees, grading and utility connections early, because moving the building later can affect several drawings at once. A designer then coordinates the construction and site information for municipal review.

After permit issuance, schedule construction and inspections in the required order. Factory building is an option, not an exemption: compare prefab backyard home methods with the site conditions. Family members and caregivers can occupy an ARU; the provincial occupancy rule does not require a family relationship. Confirm any municipal registration and occupancy steps before anyone moves in.

Related: garden suites · laneway suites Toronto · modular homes Ontario · prefab homes Ontario · basement suite vs garden suite · plans · lot check

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