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.

Measure the actual basement clearance under joists, beams and ducts. The applicable height rules depend on the existing building and proposed suite; confirm the design path with the City before pricing structural work.

Step three: confirm the escape and window requirements for the proposed unit with the City and a qualified designer. The compliant route depends on the building and the applicable Building Code provisions.

Step four: have a qualified designer specify the required fire separation and alarm arrangement for the proposed suite. Ask how penetrations will be sealed and how the finished assemblies will be inspected.

Review sound separation, electrical, heating, ventilation, plumbing and permit requirements with a qualified designer. The applicable provisions depend on the existing building and proposed 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.

For a secondary suite quote, ask for separate costs for design and permits, structural work, fire separation, plumbing, electrical, ventilation, windows, finishes and inspections. Existing conditions can change the scope.

HST: the one renovation where the rebate can be in play. Taxable renovation work supplied by an HST registrant generally carries 13 per cent HST; ordinary work does not generally qualify for the GST/HST new housing 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; authorization for compulsory electrical and plumbing trade work, 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.