Additional Residential Units (ARUs) in Ontario: How Many Units Can You Build on Your Lot?
A plain-English guide to additional residential units (ARUs) in Ontario: how many you can build as-of-right, what ARU, ADU, and garden suite actually mean, and the conditions that still apply.
What Is an Additional Residential Unit (ARU)?
An additional residential unit (ARU) is any self-contained dwelling added to a property that already has a primary home. Each ARU has its own kitchen, bathroom, and entrance, and it can take several forms: a basement apartment, a unit above or beside a garage, or a detached garden suite or laneway suite in the backyard. The term comes from Ontario's Planning Act, which is why municipalities, lenders, and incentive programs all use it. If you have heard the phrases secondary suite, in-law suite, granny flat, or accessory dwelling unit, they all describe versions of the same idea. Understanding that they are one legal family of units is the first step, because the rules for how many you can build, and the programs that help pay for them, are written around the ARU framework rather than around any single nickname.
How Many Units Can You Build As-Of-Right in Ontario?
For most residential lots in Ontario, the answer is up to three residential units as-of-right. That typically means your main house plus two additional units, such as a basement apartment and a detached garden suite. The City of Toronto goes further, permitting up to four units (a fourplex) citywide and as many as six in some districts. As-of-right means the use is allowed in principle without applying for a rezoning or a minor variance, which removes the slowest and least predictable part of the old process. It does not mean anything goes: each unit still has to satisfy the municipality's size, height, setback, lot-coverage, and servicing rules, which vary from city to city. The unit count is your starting point, and a zoning review confirms whether your specific design actually fits.
ARU vs ADU vs Garden Suite: The Terminology, Untangled
The words get used interchangeably, but they are not all the same. Additional residential unit (ARU) is Ontario's official legal term and the umbrella for every secondary unit. ADU, or accessory dwelling unit, is the broader North American term for the same concept. A garden suite is a detached ARU built in a backyard on a lot without lane access, while a laneway suite is a detached ARU that fronts a public lane. A basement apartment or secondary suite is an interior ARU inside your existing home. So a garden suite is one type of ARU, and an ARU is one type of ADU. When you are talking to your city or a lender, ARU is the term that keeps everyone on the same page.
What O. Reg. 462/24 and Recent By-Laws Changed (2024-2026)
Ontario has spent the past few years deliberately removing barriers to gentle density. Provincial reforms under the Planning Act, including O. Reg. 462/24, expanded as-of-right permissions, made garden suites possible across far more lots, and reduced obstacles like parking minimums that used to block backyard units. The City of Toronto layered its own by-laws on top, permitting multiplex housing of up to four units citywide and more in some areas. The practical effect is that projects which once required a lengthy committee approval can now proceed straight to a building permit in many cases. Because the specific dimensional rules differ by municipality and continue to be updated, treat the direction of travel as settled but always confirm the current numbers for your lot with the City.
Conditions That Still Apply: Servicing, Fire Access & Building Code
As-of-right zoning clears the planning hurdle, but every ARU still has to be a safe, legal building. Each unit needs proper water, sewer, and hydro service, which for a detached garden suite means running utilities to the rear of the lot. Detached units generally require a clear fire-access route from the street to the unit's door so emergency crews can reach it. And every unit must meet the Ontario Building Code for egress, ceiling height, fire separation, and ventilation. Some municipalities also require you to register a second unit. None of these are reasons not to build; they are simply the engineering and code realities that a competent design-build contractor folds into the plan from day one.
How to Confirm What Your Lot Allows
The only reliable way to know how many units your property can support, and in what configuration, is a zoning review tied to your actual lot. It considers your lot size and frontage, the existing structures, available backyard area, easements, and any heritage or floodplain constraints. From there you can decide whether a basement apartment, a garden suite, or both makes the most sense. Leo Constra offers a free, no-obligation feasibility assessment that pairs the zoning check with a realistic look at servicing and budget, and our garden suite cost guide and garden suite builder pages walk through what building one actually involves. ARUs also unlock real financial upside, from development-charge relief to tax credits, which we keep current on our financing page.
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