What is an additional dwelling unit (ADU)?

An additional dwelling unit is a separate dwelling on a residential lot beyond the primary house. In Ontario planning terminology, ADUs include garage-based units (often called carriage houses when newly built), laneway houses, and basement or upper-floor apartments within the same property. Municipalities also refer to these as accessory dwelling units or secondary suites depending on local policy.

Ontario's planning framework has shifted toward permitting ADUs as-of-right on many residential properties. This means that in participating municipalities, property owners can add a second or third unit without requiring a zoning variance or minor variance application. The province has encouraged this approach to address housing supply and allow homeowners to create rental income or accommodate family members on their land.

ADU rules vary by municipality. Some allow one unit, others permit two. Building codes require separate entrances, utilities, and often parking or setback requirements. Garage builders and laneway house specialists work within these local standards to construct new units or convert existing structures into compliant dwellings.

The as-of-right status makes ADUs practical for property owners looking to develop their land without lengthy approvals, though local by-laws and building permits remain mandatory.