Key takeaways
| Question | Short answer | Source |
|---|---|---|
| Self-contained home | An ARU has kitchen and bathroom facilities intended for that unit only. | ARU definition |
| Three-unit rule | Eligible urban residential lots allow three units in the house, or two plus one ancillary unit. | Planning Act unit rules |
| Parking | A zoning by-law cannot require more than one parking space per additional unit. | Parking rules |
| Who can live there | An ARU occupant does not have to be related to the main-house occupant. | Occupancy rules |
| Local review | Municipal zoning still determines height and setbacks, and a building permit is required. | Municipal requirements |
What counts as an additional residential unit
The Planning Act defines a residential unit as a self-contained set of rooms in a building or structure, used as residential premises, with kitchen and bathroom facilities intended for that unit only. The ARU rules apply on a "parcel of urban residential land": land within a settlement area where residential use is permitted and that is served by municipal sewage works and a municipal drinking water system. New to the term? Read what is an ADU.
Source: Planning Act, R.S.O. 1990, c. P.13, s. 1(1) (e-Laws).
The three-unit rule
Section 35.1(1) of the Planning Act says a zoning by-law cannot prohibit any of these on a parcel of urban residential land with a detached house, semi-detached house or rowhouse:
- Two units in the house, if all ancillary buildings together contain no more than one unit.
- Three units in the house, if no ancillary building contains a unit.
- One unit in an ancillary building (such as a garden suite), if the house contains no more than two units and no other ancillary building contains a unit.
In practice, that allows up to three units on the lot, with at most one of them in a separate building. Section 16(3) applies the same limits to official plans.
Source: Planning Act, ss. 16(3) and 35.1(1).
Parking, unit size and who can live there
- Parking. A zoning by-law cannot require more than one parking space for each additional unit (Planning Act s. 35.1(1.1)). Under O. Reg. 299/19, where the by-law does not require a space for the main unit, none is required for either additional unit, and an ARU space may be a tandem space.
- Unit size. A zoning by-law cannot regulate the minimum floor area of an additional unit (s. 35.1(1.2)).
- Who lives there. O. Reg. 299/19 says an ARU may be occupied by any person, whether or not they are related to the occupant of the main unit, and whether or not either occupant owns the lot.
- Age of the house. Where ARUs are authorized, they are permitted regardless of when the main house was built.
Sources: Planning Act s. 35.1; O. Reg. 299/19 (e-Laws).
Lot coverage, separation and angular planes (O. Reg. 462/24)
O. Reg. 462/24, filed 20 November 2024, added these standards for ARUs on parcels of urban residential land:
- Lot coverage: up to 45% of the parcel may be covered by buildings and structures. A by-law allowing more prevails.
- Floor space index: no limit, subject to the by-law's maximum height and minimum setbacks.
- Minimum lot area: the same as would apply if no ARUs were on the parcel.
- Angular planes: a building with an ARU may penetrate an angular plane in the by-law, subject to height and setback limits.
- Separation: a building with an ARU must be at least 4 metres from another building on the parcel that contains a residential unit, unless the by-law permits less.
Source: O. Reg. 462/24 (e-Laws).
What your city's zoning still decides
The provincial rules set a floor, not a design. Height, setbacks and other limits still come from your municipality's zoning by-law, and a building permit is still needed. In Toronto, for example, garden suites are regulated by Chapter 150.7 of Zoning By-law 569-2013, and section 415-6(A)(2) of By-law 1137-2022 exempts second, third or fourth units on a single residential parcel from development charges. Compare cities in garden suite rules across the GTA, and see Toronto garden and laneway suite permits and Mississauga garden suite permits.
Source: City of Toronto — Garden Suites.
Where Modular X fits
Modular X builds the detached kind of ARU: garden and laneway suites, built in modules. A feasibility check reviews your zoning, access, trees and servicing before any design or price. Start with a lot feasibility check, read the garden suite overview, or see prefab and modular garden suite rules and laneway suites in Toronto.
FAQs
Disclaimer: Figures cited are market estimates provided for information only and are not a quote. Municipal rules, fees, and programs change — always verify current requirements with your local authority. Modular X provides exact pricing only after a site-specific feasibility review.