Secondary suites and basement apartments

A secondary suite is a self-contained second dwelling unit in a house: its own kitchen, bathroom and entrance. It is the most valuable renovation available to most Ottawa homeowners and the most regulated one on this site, because the Building Code treats two households in one building very differently from one.

Step one: zoning and eligibility. Confirm the zoning permits a second unit on your lot, and check parking and entrance location with City of Ottawa planning. That is a separate question from whether the Building Code can be satisfied, and both have to be yes.

Step two: ceiling height, which decides everything. The Building Code wants a minimum clear height of 1.95 m in a basement secondary suite, and 1.85 m under beams and ducting (OBC basement suite guide). This is what kills most Ottawa basement suite projects before they start, because a 1955 bungalow in Alta Vista often has 1.90 m under the joists and less under the trunk duct. Measure it first.

Step three: egress. Every bedroom in the suite needs an egress window with an unobstructed openable area of at least 0.35 square metres, no dimension less than 380 mm, and an interior sill no higher than 900 mm above the finished floor or fixed steps (OBC basement suite guide). In a basement that means cutting the foundation wall and building a window well.

Step four: fire separation and alarms. The suite has to be separated from the rest of the house by a fire separation. For a basement suite that means a 30 minute rating on the walls and the floor assembly between units, achieved either as a rated fire separation or as a continuous smoke-tight barrier of 15.9 mm type X gypsum board on both sides. Smoke alarms have to be interconnected, by wire or wirelessly, so that one sounding sounds all of them throughout the building. Continuity is what inspectors look at hardest: one unsealed penetration defeats the assembly.

Step five: sound, services and the permit. Sound transmission between units is rated at not less than STC 43 for newer buildings, and a building more than five years old is not held to that requirement, though the separation is still worth building properly (OBC basement suite guide). Services have to be sorted too: a separate electrical sub-panel or a compliant arrangement, heating and ventilation for the suite, and a water shut-off. All of it is permit work.

How long it takes. Design, zoning check and permit is commonly two to four months. Construction of a full legal basement suite in an existing Ottawa house is three to five months, longer if the egress window excavation falls in the frozen months. Underpinning to gain height is a separate structural project and can add two to three months.

What it costs in Ottawa in 2026. Indicative ranges. A legal secondary suite in an existing basement commonly runs $80,000 to $120,000 (HomeStars Ottawa 2026). Inside that, a kitchen or kitchenette is $20,000 to $50,000, a bathroom with new plumbing $15,000 to $30,000, waterproofing and insulation $5,000 to $15,000, electrical upgrades $1,500 to $5,000, permits $500 to $1,500, and each egress window $3,500 to $6,000. Add 13 per cent HST.

HST: the one renovation where the rebate can be in play. Ordinary renovation work carries HST at 13 per cent with no rebate. A substantial renovation, which the CRA defines as removing or replacing 90 per cent or more of the interior of an existing house, can attract the GST/HST new housing rebate, as can converting non-residential property to residential use. A basement suite built inside an otherwise untouched house does not meet that test. The tax treatment of rental income is a separate question for an accountant.

What makes an Ottawa secondary suite different. Ceiling height in the post-war bungalow belt, Alta Vista, Elmvale, Carlington and Overbrook, is the binding constraint, and it is why so many of these projects become underpinning projects. Ottawa also sits largely on Champlain Sea clay, so lowering a basement floor beside an existing footing is an engineering problem rather than a matter of digging. And basement sewer backup is a real risk here, so putting a second household in the lowest part of the house is the moment to ask about a backwater valve and a sump.

Who should do the work, and what protects you. General contracting is not licensed in Ontario. What is real: the designer's BCIN, or their OAA or PEO standing; certificates of qualification for the electrician and the plumber, both compulsory trades, with an ESA notification for the electrical; an engineer's design for underpinning or for cutting the foundation wall (PEO directory); and WSIB coverage, compulsory in construction, whose occupant-retained exemption does not extend to subcontractors (WSIB). Tarion does not cover this work (Tarion exclusions). Your protections are the permit and inspections, a written contractor warranty, and the ten per cent statutory holdback (Construction Act holdback in 2026).

What to ask before you hire. What is the measured clear ceiling height, and under the ducts. Is the zoning confirmed for a second unit on this lot. How is the fire separation built, and how are penetrations sealed. Are the smoke alarms interconnected throughout the building. Where does each egress window go and who designed the opening. Is underpinning needed, and if so who is the engineer. Has ESA been notified. And get the final sign-off in writing.