Home additions and extensions

An addition is the only renovation that is really a small building project. It has its own foundation, its own structure and its own relationship to the zoning by-law, and it is the one job on this site where a design professional is not optional in practice. It is also where the ground under Ottawa starts to matter.

Step one: zoning before design. Check what the zoning by-law allows on your lot: the setbacks from each property line, the maximum lot coverage, the permitted height and the rear yard requirement. If the design needs relief from any of those, that is a minor variance application to the Committee of Adjustment, which adds months rather than weeks. A design produced before the zoning check is a design that may be thrown away.

Step two: the design and who is qualified to produce it. Permit documents in Ontario generally have to be prepared by a designer registered with the Ministry and holding a BCIN in the appropriate category, unless you are designing work on a building you own, or the designer is an architect or a professional engineer (the designer qualification rules). For a house-scale addition a qualified designer is usually enough. Once the work falls under Part 4 of the Building Code, an engineer's design is required, as it is for a retaining wall over 1,000 mm of exposed height. Verify an engineer in the PEO directory.

Step three: the foundation, and the clay. A large share of Ottawa sits on Champlain Sea clay, often called Leda clay, which swells when wet, shrinks as it dries and can lose much of its strength when disturbed (NRC). Two consequences. Foundations in clay have to go deep, as much as 1.2 m below grade under the Building Code. And a new foundation beside an old one on clay is a differential settlement problem, which is why the joint between the addition and the house is where it shows. A geotechnical review is money well spent on any addition of size.

Step four: locates, excavation and the shell. Before any digging, a locate request through Ontario One Call is mandatory in Ontario, it is free, and it takes at least five business days. Then excavation, footings, foundation walls, waterproofing and backfill, followed by framing, roof, windows and the envelope. Getting the shell closed in before winter is the milestone that matters in this climate, and a project that reaches November without being closed in will lose weeks.

Step five: tie-in, services and finishes. The addition is tied into the existing roof, wall and floor structure, and the heating, electrical and plumbing are extended. This is usually where an older Ottawa house reveals that the furnace and the electrical panel were sized for the original footprint, and a panel upgrade or a second heating zone appears on the budget.

How long it takes. A single-storey rear addition of 20 to 35 square metres is typically four to seven months from permit issue. A second-storey addition is six to twelve, because the roof comes off and the house has to be weather-protected while it does. Add two to five months in front of that for design, zoning and permit.

What it costs in Ottawa in 2026. Indicative ranges. A single-storey rear addition on a new foundation commonly runs $3,500 to $6,000 per square metre of new space in this market, so a 30 square metre family room addition is roughly $105,000 to $180,000. A second-storey addition typically runs higher per square metre because of the temporary weather protection and the structural work below. Design, engineering and permit fees together are commonly five to twelve per cent of construction cost. Add 13 per cent HST, which is not rebatable on renovation work unless the project is a substantial renovation of 90 per cent or more of the interior.

Warranty, and where Tarion stops. The Tarion new home warranty covers newly built homes, including a contract home built for you on land you own, from a builder licensed by the HCRA, with one, two and seven year periods. It does not cover renovation of an existing home, and it excludes work and materials supplied by the homeowner (Tarion exclusions). An addition to a house you already live in is a renovation, so ask for a written warranty and check what it covers, particularly the envelope and the roof tie-in.

Who to hire and what to verify. General contracting is not licensed in Ontario. What is verifiable: the designer's BCIN or their OAA or PEO standing, certificates of qualification for the electrician and the plumber, ESA notification for the electrical, and the contractor's WSIB status, compulsory in construction with an exemption for occupant-retained work that does not extend to subcontractors (WSIB).

What to ask, and what to hold back. Has the zoning been checked, and does this need a minor variance. Who produces the permit drawings and what is their qualification. Is there a geotechnical report, and what foundation depth is designed. Who is the engineer of record. What is the schedule for closing in the shell. Is the panel or furnace being upgraded, and is that in the price. On payment, hold back ten per cent of the price of services and materials as supplied, and note that from 1 January 2026 the release of accrued holdback runs on an annual notice schedule (Construction Act holdback in 2026). That holdback is your protection against a lien from a subcontractor the general did not pay.