Terminology

Same idea, a dozen names.

ADU, ARU, second suite, in-law suite, garden suite, laneway house: everyone researching this ends up confused by the terminology before they’ve even started. Most of these words describe the exact same thing. Here’s what each one actually means, and which one your municipality uses on the permit application.

ADU
ARU
Second suite
Garden suite
Laneway house

Show me the key terms Ask about my property

Term translator

9 names. One permit term.

Pick the word you’ve heard. We’ll show you what it means on paper.

You heard

ADU

Additional Dwelling Unit

Search term

An ADU is a second, self-contained home on a property that already has one. It’s the term most people search for, not an official Ontario term.

Who says it: What people type into Google
Where it sits: Anywhere on the lot
On your permit: ARU

What’s the difference between an ADU and an ARU?

Nothing, in practice. ADU is the term people search for; ARU is Ontario’s official term, the one on your permit application.

ADUEveryday term

Additional Dwelling Unit

An ADU is a second, self-contained home on a property that already has one. It’s the term most people search for, not an official Ontario term.

ARUPermit term

Additional Residential Unit

An ARU is Ontario’s official term for an additional unit on a residential lot. It’s the word your zoning bylaw and building permit application use.

We use ADU because it’s what people look for, and ARU because it’s what gets typed on your permit.

Where can an additional unit go?

Inside the house, attached to it, or in a separate building on the lot. Most location terms fit into one of these three groups.

See the difference in one glance.

Inside the house

Basement apartment

A basement apartment is a self-contained suite in the basement, with its own kitchen, bathroom, sleeping area and entrance. It’s the most common interior unit.

Backyard

Garden suite

A garden suite is a detached unit in the backyard. It’s the general term for a standalone backyard unit, and it doesn’t need to face a laneway. Also called a coach house when it’s built above or beside a garage.

Lot with a laneway

Laneway house / laneway suite

A laneway house is a detached unit on a lot that backs onto a public laneway, accessed from that lane. In effect, it’s a garden suite that faces a lane.

Everyday language, same meaning.

Everyday

Second suite / secondary suite

A second suite is everyday language for any additional unit, interior or detached. In normal conversation it means the same as ADU or ARU.

Casual

In-law suite / granny flat

An in-law suite is an additional unit described by its use: built or set aside for a parent or relative rather than a paying tenant. Physically, it’s the same as any second suite.

Older bylaw

Accessory apartment

An accessory apartment is older municipal bylaw language for an additional unit. Ontario’s provincial framework now calls it an additional residential unit.

The words on the paperwork.

As-of-right: As-of-right means allowed under the existing zoning bylaw, with no rezoning application or committee hearing. Ontario requires most urban residential lots to permit up to three units (main house, basement suite, garden suite) as-of-right.
Egress window: An egress window is a window big enough for a person to climb out and a firefighter to climb in. The Building Code requires at least one on every floor level with a bedroom, unless a door on that level leads outside.
Fire separation: A fire separation is the fire-rated wall and ceiling assembly between an interior unit and the rest of the house. It contains a fire long enough for both households to get out.
Setback: A setback is the minimum distance required between a building and the property line. It decides whether a garden suite fits on a lot, and where.
Servicing: Servicing is connecting a detached unit to water, sanitary sewer and hydro, usually through new lines trenched across the yard. It’s the main reason garden suites cost more than basement conversions.
BCINBuilding Code Identification Number: A BCIN is the Building Code qualification held by the person or firm preparing your permit drawings, unless you draw your own house. Ask for it, and check it on Ontario’s public QuARTS registry, before hiring anyone.