ADU vs Coach House vs Garden Suite
Ottawa Definitions

A Kanata couple came to us in early 2026 wanting to build "an ADU" for an aging parent. We asked which type. They said "the small house in the back yard." That is a coach house. They said "or maybe just a basement apartment." That is a secondary dwelling unit. They said "or one of those small detached cottages we saw in Toronto." That is a garden suite. The terms get used interchangeably online. They are not interchangeable in Ottawa zoning. The wrong word in your permit application can route your file to the wrong reviewer and add weeks of confusion.

Here is the actual Ottawa terminology, what each term means under the City's by-laws and the Planning Act, and which one fits the project you are describing.

The Umbrella Term: ADU

"ADU" stands for Accessory Dwelling Unit. It is a planning-and-policy umbrella term that originated in U.S. zoning practice and migrated north. In Ontario, ADU is not a strict legal term in the Planning Act or in Ottawa's zoning by-law. It is shorthand for "any additional dwelling unit on a lot that already has a primary house."

Under the umbrella, three subtypes show up in Ottawa:

• A unit inside the main house (basement apartment, attic conversion, attached secondary suite)
• A unit attached to the main house (above-garage suite with separate entry)
• A unit in a separate building on the same lot (coach house, garden suite, laneway suite)

The Ontario Planning Act since 2019 (and reinforced by Bill 23) requires every municipality to permit up to three units on any lot zoned for residential. Ottawa implements this through By-law 2008-250. So "is an ADU allowed on my lot?" is almost always yes, but which subtype is allowed depends on the zone.

Secondary Dwelling Unit (SDU): The Inside Suite

The Secondary Dwelling Unit is the basement apartment, the attic conversion, or the attached suite carved out of an existing house. In By-law 2008-250 and the Ontario Building Code, this is the most regulated and best-understood category.

Key requirements:

• The unit must be self-contained: own kitchen, own bathroom, own entry (separate or shared at-grade)
• The fire separation between units must be 45-minute or 1-hour per OBC 9.10.9.14, depending on whether sprinklers are installed
• Egress windows in basement bedrooms per OBC 9.9.10 (minimum 0.35 m² opening, no dimension less than 380 mm, sill below 1500 mm)
• Smoke alarms hardwired and interconnected per OBC 9.10.19
• Minimum ceiling height typically 1.95 m (6 ft 5 in) over at least 75% of the area
• Parking: minimum one space, sometimes shared with the main unit per zone
• Registration with the City of Ottawa as a secondary dwelling unit

SDUs do not require site plan control in most cases. The building permit covers fire, egress, and energy compliance. The zoning side is usually as-of-right, particularly under the Bill 23 three-unit allowance.

Coach House: The Detached Building

The coach house is a separate building on the same lot, accessed from a laneway, side yard, or rear yard. The term has historical roots: the original coach houses were small structures behind the main house that held carriages and horses, with quarters for staff above. Modern coach houses are full dwelling units on their own footprint.

Ottawa's by-law calls these "coach houses" in some sections and "detached secondary dwelling units" or "detached additional dwelling units" in others. The legal definition has tightened since 2018:

• Maximum gross floor area: typically 80 m² to 95 m² depending on zone (varies by subzone)
• Maximum height: typically 6.5 m to 7.5 m, sometimes capped at one storey of useable space above grade
• Setbacks: minimum 1.0 m from rear and side property lines, sometimes more
• Lot coverage: combined with main building must stay within zone cap (typically 40-55%)
• Servicing: separate or shared water, sewer, electrical (engineering required for service splits)
• Site plan control: often triggered, depending on lot specifics and zone

The detached nature of a coach house is what makes it different from an SDU. You are erecting a new building, not modifying the existing one. The full implications of that (foundation, services, drainage, tree impact, often heritage review) are why coach house permits run longer and cost more.

Garden Suite: A Special Subtype

Garden suite is a subtype that gets confused most often. Under Ontario's Planning Act and the City of Ottawa's policies, a "garden suite" historically meant a temporary detached unit, often modular, intended for a specific occupant (like an aging family member), with a time-limited authorization that expired when the occupant left.

The traditional garden suite was approved by way of a temporary use by-law for periods of up to 20 years, after which it was supposed to be removed. This was and is a separate process from a permanent coach house.

In current Ottawa terminology, the line has blurred. Many homeowners now use "garden suite" to mean any small detached dwelling in the back yard, intended as a permanent structure. The City still has the temporary use pathway available, but most projects today are pursued as permanent coach houses or detached additional dwelling units.

If you are exploring a garden suite for a temporary need (a family member, a few years), the temporary use pathway can be cheaper and faster. If you want a permanent structure, file it as a coach house from day one.

Laneway Suite: An Imported Term

"Laneway suite" is mostly a Toronto term. Toronto's zoning carved out a specific laneway-suite typology with its own rules. Ottawa does not formally use the term in the by-law. What Toronto calls a laneway suite, Ottawa calls a coach house.

This matters when you are reading online resources. Toronto Star articles on laneway suites, Toronto blog posts, and Ontario-wide blog content all use Toronto's terminology. The rules they cite (size caps, parking, lane access requirements) are Toronto rules and do not apply in Ottawa. Always cross-check Ottawa-specific by-law language before assuming your project follows the same template.

Comparison at a Glance

For a typical Ottawa urban lot, the four common ADU types compare as follows:

• Basement SDU: 60-110 m² inside existing house, lowest cost of the four because the shell is already there, 4-8 month timeline, building permit only
• Above-garage attached suite: 40-90 m² addition, mid-range cost driven by new framing and a separate entry, 6-10 month timeline, building permit and sometimes minor variance
• Coach house (detached): 60-95 m² new building, highest cost because it is a full new build with its own foundation, services, and envelope, 9-15 month timeline, building permit plus often site plan
• Garden suite (temporary use): 40-80 m² modular or built-in-place, cost varies with whether the unit is modular or stick-built, 8-12 month timeline, temporary use authorization plus building permit

Cost-per-square-foot is highest for coach houses because you are paying for a new foundation, new services, new envelope, and a full new build. It is lowest for basement SDUs because the shell is already there. We provide a detailed written breakdown once the path and scope are set, so you see exactly what drives the number on your lot.

"The right word in your permit application sends the file to the right reviewer. The wrong word sends it on a tour of three departments before it lands. Naming matters."

Which One Fits Your Goal?

Three questions narrow the answer:

1. Permanent or temporary? Permanent leans coach house or SDU. Temporary leans garden suite.
2. Attached or detached? Attached or interior leans SDU. Detached leans coach house or garden suite.
3. Existing house big enough? If yes, SDU is cheapest. If no or you want privacy separation, detached is the play.

For most aging-in-place situations where the parent will live in the unit for 5-15 years and the family wants the option to convert to rental afterward, we recommend coach house. The capital cost is higher, but the asset is permanent and increases the property value.

For maximum rental yield on an existing house, basement SDU is usually the answer. Lower cost, faster permit, and the income covers a meaningful chunk of mortgage carrying costs.

Bill 23 and the As-of-Right Three Units

Since Bill 23 (the More Homes Built Faster Act, 2022), every Ottawa lot zoned for residential is permitted up to three units as-of-right, in any combination of the above types. The combinations are typically:

• Main unit + basement SDU + coach house
• Main unit + attached SDU + coach house
• Three units inside the main house (rare but possible in larger homes)

"As-of-right" means the zoning permits it without a variance, but you still need a building permit and you may still trigger site plan control. The zoning is the easy part; the engineering, fire separation, and servicing are the work.

Where We Start

For any Ottawa client exploring an ADU, the conversation starts with three pieces of information:

1. The lot's zoning subzone and any exceptions
2. The existing house's structure and capacity to host an SDU
3. The available space on the lot for a detached unit (setbacks, trees, services)

From there, we map the by-law against the project goals and recommend a path. Sometimes the answer is "do the basement first, then add a coach house in five years." Sometimes it is "skip the basement, go straight to coach house, the lot is right." We can run this assessment as part of our free 48-hour feasibility review. Call 613-862-4555 or read more about our Ottawa coach house design and build process.