Secondary Dwelling Units in Ottawa
The Full Guide

A secondary dwelling unit in Ottawa is a self-contained apartment on the same lot as the main house, inside it or in a coach house. The City allows up to two on a lot with city water and sewer, either two apartments in the house or one apartment and one coach house, for no more than three units in total.

Adding one needs a building permit even when little construction is planned. Since Ontario's 2024 Building Code took effect on 2025-01-01, a house with one secondary suite can use a continuous smoke-tight barrier of 15.9 mm Type X drywall in place of a rated fire separation.

Half the basement apartments in Ottawa are technically illegal. The owners did not break the law on purpose. They finished a basement, added a kitchenette, rented it out, and never asked the City whether the unit was legally separate from the main house.

Then a tenant complains, the City inspector knocks, and the conversation becomes uncomfortable. Or the owner tries to refinance and the appraiser flags the second unit. Or the buyer's lawyer asks for proof of legal status during a sale.

Bill 23 made it easier than ever to do a secondary dwelling unit (SDU) properly. Here is the full picture: what counts as an SDU, what the OBC actually requires, what it costs, and how to legalize a basement that has been quietly rented for ten years.

What Counts as an SDU in Ottawa

A secondary dwelling unit is a self-contained residential unit, separate from the principal dwelling, with its own kitchen, bathroom, sleeping area, and entry. The City's definition requires:

• Sleeping area
• Cooking facilities (kitchen sink + cooktop or range + counter space)
• A 3-piece bathroom at minimum (toilet, sink, tub or shower)
• Fire separation from the principal dwelling
• An entrance that does not require passing through the principal dwelling

If any of these are missing, you have an "in-law suite" or a finished basement, not an SDU. That distinction matters because the legal requirements and the rent income are different.

The Three SDU Configurations

Basement Apartment

The most common Ottawa SDU. Existing basement is converted to a self-contained unit accessed by either an interior stair behind a fire-rated door or a separate exterior entrance. About 70% of the SDU work we do falls into this category. The investment to legalize a basement that already has rough framing depends on what is already there, with finishes, plumbing, and egress work driving most of the range.

Coach House (Detached SDU)

A separate building in the rear yard, often above or beside an existing detached garage. Maximum 80 m² gross floor area in Ottawa. Maximum height 6.5m. Coach houses need their own water and sewer connections, electrical service (or sub-panel from the house), and full OBC compliance.

A new build is a substantially larger investment than converting an existing garage with attic space, since new servicing and a full envelope drive the cost.

Updated 2026-10-05: Ottawa's new Zoning By-law 2026-50 sets different coach house limits from the figures above. They are listed under the 2026 rules further down this page.

Garden Suite

Similar to a coach house but smaller and often single-storey, a freestanding unit detached from the principal dwelling. Same 80 m² cap. The investment is set by site servicing, foundation type, and finish level. Ottawa has been approving more garden suites since Bill 23 because they offer a senior-living option without selling the house.

The Ontario Building Code Requirements

Every legal SDU has to satisfy the OBC, specifically Part 9.36 for energy efficiency, Part 9.10 for fire protection, and the relevant egress and ventilation sections. The five non-negotiables:

1. Egress

Every habitable room and bedroom needs an exit window or door that meets OBC 9.9.10.1: minimum clear opening of 0.35 m², no dimension less than 380mm, and a sill not more than 1m above the floor in bedrooms. The window well, if used, has minimum dimensions of 0.55 m² with 760mm clear from the window to the wall opposite.

Most unfinished Ottawa basements need new egress windows cut into the foundation, a cost driven by concrete cutting, the lintel, the window, the well, and drainage per opening.

2. Fire Separation

The SDU has to be separated from the principal dwelling by a continuous 45-minute fire separation. In practice this is type X drywall (5/8" Type X) on both sides of the dividing assembly, including the underside of the floor above the SDU and any shared walls. Penetrations (ducts, plumbing, wiring) need to be firestopped. Existing basement ceilings often need to be opened up to upgrade the assembly.

3. Smoke Alarms

Each unit needs interconnected smoke alarms in every bedroom, in the area outside bedrooms, and on every storey. The alarms must be hardwired with battery backup and interconnected so all alarms in both units sound when one detects smoke. CO alarms are required wherever there is a fuel-burning appliance or attached garage.

4. Sound Transmission

The dividing assembly needs to meet STC 50 for airborne sound. Practically this means insulation in the joist cavity above the SDU, resilient channels or staggered studs, and properly sealed perimeters. Tenants who can hear the family above them at conversation volume create the kind of complaints that bring the City inspector to the door.

5. Ventilation

Each unit needs its own continuous mechanical ventilation system (typically an HRV) sized to the unit's floor area. Bathroom and kitchen exhaust fans on their own do not satisfy this. The ventilation system has to be designed by someone qualified and the design submitted with the permit.

What the 2024 Building Code Changed for a House With One Secondary Suite

Ontario's 2024 Building Code, in force since 2025-01-01, rewrote the rules for a house with exactly two units. The Ministry of Municipal Affairs and Housing summarized the changes in its August 2024 Part 9 briefing:

• A secondary suite is a self-contained unit in a residential building that holds only one other unit plus common spaces, with both units forming a single real estate entity (Division A, Article 1.4.1.2). Ontario set no size limit on the suite.
• Under Article 9.10.9.16, one unbroken layer of 15.9 mm (5/8 inch) Type X drywall over the shared walls and the floor-ceiling framing can stand in for a fire separation between the two units.
• Ceilings in the suite must be at least 1.95 m high, with 1.85 m clear under beams and ducts (Article 9.5.3.1). The same 1.85 m applies over stairs that pass under beams and ducts (Article 9.8.2.2).

Our guide to Ontario fire code basics for home renovations shows how the barrier goes in, along with the wireless smoke alarm option for two units on separate electrical services. A third unit puts the house outside the secondary suite definition, and the full separation rules for dwelling units return.

Permits, Trades, and Inspections

An SDU needs:

1. Building permit through MyServiceOttawa, with BCIN-stamped drawings
2. ESA electrical permit (separate from the building permit)
3. Plumbing permit if you are adding fixtures (often bundled with the building permit at the City)
4. HVAC review if you are adding a separate furnace or HRV
5. TSSA gas permit if you are adding a gas line for a new range or dryer

The inspection sequence is the same as any renovation but tighter on fire separation and egress: footing if there is new concrete, underground plumbing, framing (with fire separation specifically inspected), insulation, drywall covering the fire separation, and finals on building, electrical, and plumbing.

For coach houses you also need a site servicing review (since you are adding a new building) and often a rear-yard drainage plan to satisfy lot grading.

Parking After Bill 23

Pre-2022, the parking minimum killed many Ottawa SDU projects. The City required a separate parking space for the second unit, which on most older lots meant paving the front yard or losing the back garden. Bill 23 removed the parking requirement for the second and third unit.

You still need parking for the principal dwelling per the by-law (typically 1 space). The SDU does not add to that minimum. Driveway-width restrictions still apply.

What the City and the 2026 Zoning By-law Allow

The City of Ottawa's page on adding an apartment (updated 2026-07-29) sets these conditions:

• On a lot with city water and sewer: up to two additional units, either two apartments in the house or one apartment plus one coach house, three units in total.
• On a lot with a well or septic system: one additional unit or a coach house, once the Ottawa Septic Office confirms the system can take the load.
• Two apartments inside the house need a designer registered under a Building Code Identification Number (BCIN). A homeowner can take design responsibility for one unit where none exists yet, or for a coach house.
• The new unit needs its own access on the ground floor unless the building or fire code allows another route.
• No extra parking space is required. A new space cannot go in the front yard, and tandem parking in the existing driveway is allowed.
• The City's target for the first permit review is 15 business days.

Units built before 2005 in Cumberland-Urban, Gloucester, Nepean-Urban, Osgoode, Goulbourn and West Carleton, where second units were already permitted, still have to meet the Ontario Building Code and the Fire Code.

Zoning By-law 2026-50, enacted 2026-03-11, adds its own limits. In the N zones it counts additional units and coach houses within each zone's unit maximum, and on a serviced lot it allows four units in a building where the density table would allow fewer (Section 801).

The City's memo to Council of 2026-07-07 lists the number of units permitted in the N zones among the provisions still under appeal at the Ontario Land Tribunal. The City applies the stricter of the old and new by-laws while both stand, so confirm the count for a specific address with the City's development information officers at 613-580-2424, extension 23434.

For a coach house in Ottawa, Section 701 of the new by-law sets the shape. On a lot under 0.4 hectares it goes in the rear yard.

The building is capped at 3.6 metres high in the urban area, sits at least 0.6 metres from the side and rear lot lines and 1.2 metres from other buildings, and its footprint, together with any accessory buildings in the same yard, cannot exceed half of that yard, to a maximum of 95 square metres. Its roof cannot hold a terrace.

A window facing a lot line needs the wall 1.2 metres back with the sill at least 1.6 metres above the floor, or the wall 4 metres back.

Sources: City of Ottawa, Adding an apartment (additional dwelling units) and Zoning By-law 2026-50 (Part 7, Part 8) and its memo to Council on the appeals (2026-07-07); Ontario Ministry of Municipal Affairs and Housing, 2024 Building Code, Part 9 briefing. All read 2026-10-05.

The Cost Drivers, Line by Line

For a typical Ottawa basement legalization where the basement is already partially finished, the scope breaks down into these line items. We itemize each one with allowances in a detailed written proposal:

• Design + permit: BCIN drawings, application fees, and City review
• Egress windows: concrete cutting and well per opening, usually 1-2 windows needed
• Fire separation upgrades: opening existing ceilings, Type X drywall, and firestopping penetrations
• Sound insulation: driven by the STC 50 assembly required
• Plumbing rough-in for kitchen + bath: depends on slab break and stack location
• Electrical rough-in + ESA inspection: service capacity and metering approach
• HVAC for separate ventilation: sized to the unit's floor area
• Kitchen install: cabinets, counter, and appliances, a big swing item on your selections
• Bathroom install: fixtures, tile, and finish level
• Flooring, paint, finishes: set by the material allowances you choose
• Separate exterior entrance (if needed): concrete steps, weather protection, and door

What moves the total is the starting point. A basement already very close to compliant sits at the low end; starting with raw concrete and joists pushes it higher. We size your written proposal to your actual basement, not a generic average.

For our complete process and transparent pricing on multi-unit conversions, see our multi-unit renovation page.

The Income Math

A clean, legal one-bedroom basement apartment commands steady market rent in 2026 Ottawa. A two-bedroom basement, if the layout allows, rents higher. Coach houses and garden suites at 80 m² command a premium because tenants treat them as proper homes, not basements.

The income math on a legal basement suite works out like this:

• Gross annual rent: the unit's monthly rent over twelve months
• Operating costs: utilities split, insurance increase, and a vacancy allowance
• Net annual income: gross rent less operating costs
• Simple payback: net income measured against the renovation investment, typically a handful of years

That math ignores property appreciation as a multi-unit property and tax-deduction effects on rental income, both of which improve the return.

"A legalized SDU is the only renovation we do where the project pays itself back inside the warranty period and adds permanent rental capacity to the housing stock."

Legalizing an Existing Illegal Basement

This is sensitive territory. If you have been renting a basement for years without permits, the path forward is:

1. Engage a BCIN designer to assess current condition against OBC and zoning
2. Stop renting (or move to a clear month-to-month understanding) before the inspector visits, since City inspections are typically prompted by complaints
3. Apply for a building permit to bring the unit to code
4. Open up ceilings and walls as needed to install fire separation, egress, smoke alarms, and ventilation
5. Pass all inspections, get the closing letter, then re-rent the now-legal unit

The City of Ottawa generally cooperates with owners who voluntarily come forward to legalize, especially since Bill 23. The alternative (waiting for an inspector to issue an Order to Comply or an Order to Vacate) is significantly worse.

Tenants and the Residential Tenancies Act

The moment you have a legal SDU and a tenant, the Residential Tenancies Act applies. Standard lease form, rent control on most units (annual guideline increases set by the Province), restrictions on terminating tenancies. The RTA also applies to existing illegal units, so legalizing does not change the landlord-tenant relationship.

If you are uncertain about RTA compliance, talk to a paralegal or landlord-tenant lawyer before listing the unit. The mistakes are expensive at the Landlord and Tenant Board.

The Decision Framework

An Ottawa SDU project makes sense when:

• Your basement, garage, or rear yard has the physical space to comply with OBC and zoning
• You can carry the construction cost (savings, HELOC, refinance)
• Your principal dwelling has separate utilities or you accept utility-split arrangements
• You are comfortable being a landlord
• The local rental market supports the rent your unit could command

It makes less sense if you are planning to sell within three years (the renovation rarely pays back fast enough through resale uplift alone), if your zoning has heritage or conservation overlays that complicate the work, or if your household cannot tolerate sharing the property with a tenant.

For a free 48-hour feasibility review on whether your specific lot can carry an SDU, what configuration would work, and a written cost estimate, call 613-862-4555 or use the contact form.

Frequently asked questions