Home types and construction methods
What is an ADU? Backyard suites in Canada explained
By Jonathan Kennedy, cabin owner and tiny home rental owner · Last reviewed October 28, 2026
Illustrative photoAn ADU (accessory dwelling unit) is a second, self-contained home on the same lot as a house, either inside it as a secondary suite or in its own building in the yard as a garden suite, laneway home, coach house or carriage house. In Ontario and BC, provincial law now requires most towns to allow one; in Alberta each municipality decides. A site-built backyard ADU costs $180,000 to $400,000 by our 2026 estimate.
What counts as an ADU in Canada?
Any second home on a house's lot with its own kitchen, bathroom and entrance. Canadians call it different things depending on where it sits and where they live:
| Name | What it is | Where you'll hear it |
|---|---|---|
| Secondary suite, in-law suite | A unit inside the house, often in the basement | Across Canada |
| Garden suite | A detached unit in the backyard | Ontario (Toronto, Ottawa) |
| Laneway home or suite | A detached unit on a lot that backs onto a lane | Toronto, Vancouver, Ottawa |
| Coach house | A detached unit, often above or beside a garage | Ottawa, Markham and other Ontario towns |
| Carriage house | A detached unit, often above a garage | BC, Alberta |
| Garage suite, backyard suite | A unit above or in a garage, or in the yard | Alberta (Calgary, Edmonton) |
| Additional residential unit (ARU) | The legal term in Ontario's Planning Act | Ontario |
This guide is about the detached kind: an ADU in its own building.
Can I build an ADU on my lot?
In most cities, yes. The rules come from your province first, then your municipality.
- Ontario: the Planning Act stops municipalities from banning one unit in a building ancillary to a house on serviced urban land, up to three units per lot (Planning Act s. 35.1). 299 of the 307 Ontario municipalities we researched allow a backyard suite, some with limits.
- British Columbia: since Bill 44 (2023), every municipality must allow a secondary suite or a detached accessory dwelling in single-family zones, and larger towns must allow three to six units on many lots. See the BC rules.
- Alberta: no provincial law requires backyard suites. Edmonton lists backyard housing as a use in residential zones; Calgary allows backyard suites as a permitted use in the R-G district and as a discretionary use elsewhere. See the Alberta rules.
What are Ontario's rules for a backyard ADU?
On a serviced urban lot, provincial law sets a floor that your town can't go below (Planning Act; O. Reg. 299/19):
- No minimum size. A by-law can't set a minimum floor area for the unit.
- One parking space at most. A town can't require more than one space for it, and none if the house needs none.
- Lot coverage up to 45% for all buildings together.
- Anyone can live there. The tenant doesn't have to be family.
- No development charges or parkland fees for the unit.
Your town still sets height, setbacks and the maximum size.
How big can a backyard ADU be?
It's set locally, and it varies a lot. Here are the limits in Ontario's largest municipalities that cap the size, quoted from each by-law:
| Municipality | Answer | Maximum size |
|---|---|---|
| Toronto | Allowed | Garden suite: 120 m² gross floor area, or 60 m² if one storey, and less than the gross floor area of the main house (150.7.60.50(2) and (4), By-law 849-2025). Lot coverage: 45% for all buildings, or the suite is excluded and all ancillary buildings together are capped at 20% of the lot (150.7.60.70(1)). Laneway suite: gross floor area less than the main house's, maximum 10.0 m long and 8.0 m wide (150.8.60.50(2), 150.8.60.30(5)-(6)) [source] |
| Mississauga | Allowed | Lots of 550 m² or less: 55 m² floor area (one storey). Lots over 550 m²: 10% of lot area, up to 100 m². Two storeys only on lots over 750 m² (and on through lots, up to 100 m²). The suite's lot coverage may not exceed the primary dwelling's; lot coverage on the lot may be the greater of 45% or the zone limit (Table 4.1.1.14, Articles 4.1.1.6, 4.1.1.9, 4.1.1.11) [source] |
| Brampton | Allowed | By-law 14-2026: gross floor area up to 100 m² with a detached dwelling, 50 m² in all other cases (s.5.2.C.15.b). City garden suite page (By-law 270-2004 rules): 80 m² in Hamlet, Estate or Agricultural zones, 35 m² in all other residential zones. The more restrictive figure applies [source] |
| Hamilton | Allowed | Gross floor area: the lesser of 75 m² or the gross floor area of the principal dwelling. If the principal dwelling's ground floor area is 105 m² or less, the D-ADU's ground floor area may not exceed 70% of it. Combined lot coverage of all buildings on a lot with a D-ADU is capped at 45% (s.4.33.2 i) [source] |
| London | Allowed with limits | Under Interim Control By-law CP-1600-300, an ARU cannot exceed 80% of the gross floor area of the primary dwelling. A detached ARU also counts toward the zone's maximum lot coverage (s.4.37(4)(f)) [source] |
| Markham | Allowed with limits | Lots accessed from the front or exterior side yard: 15 m² GFA above grade per accessory building if lot area is 500 m² or less; 60 m² if lot area is over 500 m² (s.4.8.1.2 b). Lane-accessed lots: 15 m² per building, but one accessory building may have lot coverage of the greater of 15% or 40 m² (lots under 4,000 m²) / 60 m² (4,000 m² or more) (s.4.8.1.3 c–d) [source] |
| Kitchener | Allowed | Building footprint not to exceed 80 m², and must comply with the maximum lot coverage in Table 7-2, 7-3 or 7-4 (s.4.12.3 e). The City guide notes coverage is typically 15% for all accessory structures. [source] |
| Windsor | Allowed | Maximum gross floor area of the additional dwelling unit in an accessory building: 100 m², inclusive of a basement or cellar (s.5.99.80.5 b) 7) [source] |
How much does an ADU cost in Canada?
| Type | 2026 estimate | What's not included |
|---|---|---|
| Site-built garden or laneway suite | $180,000 to $400,000 | Varies by city; Toronto figures are the baseline |
| Prefab or modular garden suite (Ontario) | $150,000 to $300,000 | Delivery, crane and site work |
These are our house figures, built from municipal and industry sources; the cost estimator shows each source and adds servicing, permits and foundations.
Our take: a prefab suite saves time on site more than it saves money overall. Get a quote that includes delivery, the crane, the foundation and the service connections before you compare it with a site-built suite; that's where the gap usually closes.
Who builds ADUs?
Garden suite builders, prefab suite makers and design-build firms. Compare garden suite builders in Toronto, laneway house builders in Vancouver, backyard suite builders in Calgary or prefab and modular builders in Ontario.
Questions people ask
- What does ADU stand for?
- Accessory dwelling unit: a second, self-contained home on the same lot as a house. In Ontario the legal term is additional residential unit (ARU).
- Is a garden suite the same as an ADU?
- A garden suite is one kind of ADU: a detached unit in the backyard. Secondary suites inside the house, laneway homes and coach houses are ADUs too.
- Do I need a building permit for an ADU?
- Yes. A detached ADU is a building with a kitchen and bathroom, so it needs a building permit in every province we cover, plus zoning approval if your lot doesn't already allow one.
- Can I rent out an ADU?
- In Ontario, yes: provincial rules say anyone may live in an additional unit, related or not. Short-term rental rules are set separately by each municipality.
- How much does a prefab ADU cost?
- A prefab or modular garden suite costs about $150,000 to $300,000 in Ontario by our 2026 estimate, before delivery, crane and site work. Our cost estimator adds those extras.