Retaining walls, steps and raised patios

A retaining wall is the only garden element that is genuinely structural. It holds back soil and water, it is loaded every day, and when it fails it fails slowly for two winters and then suddenly. It is also where Ontario's rules stop being advisory.

Step one: the height, because it decides the whole project. A permit is required where a retaining wall exceeds 1,000 mm of exposed height and is adjacent to public property, building access, or private property open to the public, and a professional engineer licensed in Ontario then has to provide a design confirming compliance with Part 4 of the Building Code. A guard is required where the drop exceeds 1.0 m, or 600 mm along a raised walkway (retaining wall permit rule). Ottawa's own guide states the threshold plainly: retaining walls over 1 m need a building permit (City of Ottawa guide). Verify your engineer in the PEO directory.

Step two: locates and excavation. A locate through Ontario One Call is mandatory before digging, free, and needs at least five business days. Then the excavation: a base trench wider than the block and dug to below the level that frost will disturb, which on a clay site means deeper than the brochure diagram.

Step three: the base and the drainage behind the wall. Two things fail walls in this city, and both are behind them. Water pressure is the first: a retaining wall needs a drainage layer of clear stone against its back face and a perforated drain at the bottom taken to a daylight outlet, so water leaves rather than pushing. Frost is the second: saturated clay behind a wall expands when it freezes and pushes the wall outward, one winter at a time. Clear stone, geotextile between the stone and the clay, and a functioning drain are what prevent it, and they are exactly the items a low quote leaves out.

Step four: building the wall. Levelling course compacted and checked, blocks set with the specified setback, geogrid layers tied back into the retained soil at the intervals the design calls for, backfill in lifts and compacted in stages rather than all at once. Geogrid is the item most often skipped and the item a wall over a metre depends on entirely.

Step five: caps, steps and guards. Caps adhered, steps built with consistent risers because inconsistent ones are the commonest trip hazard in a garden, and a guard where the code requires one. Steps in a public-facing location should follow the rise and run the Building Code sets rather than whatever the slope produced.

How long it takes. A garden wall under 600 mm with steps is three to six days. A wall of 1,000 to 1,500 mm with geogrid, drainage and a raised patio behind it is two to four weeks on site, plus two to six weeks in front of that for the engineer's design and the permit. Build between May and October: a wall backfilled against frozen clay is a wall built on an unknown.

What it costs in 2026. Indicative ranges. Segmental block walls in this market commonly run $60 to $120 per square foot of exposed wall face installed, which puts a 20 m run at 1 m high at roughly $13,000 to $26,000, with natural stone and armour stone higher again. Steps are commonly $250 to $600 per step depending on material and whether they are freestanding. A raised patio behind a wall is priced as wall plus paving, with paver patios at $25 to $45 per square foot installed (RenoQuotes 2026). An engineer's design for a wall over a metre is typically $1,500 to $4,000, and the permit is on top. These are market ranges rather than figures from a primary source. Add 13 per cent HST.

What makes an Ottawa wall different. Clay and frost together. Champlain Sea clay swells when wet, shrinks as it dries and loses strength when disturbed (NRC on Leda clay), so the retained soil behind a wall is both heavy and prone to holding water. The Building Code's foundation depths of as much as 1.2 m below grade in clay soils tell you how far frost reaches here (OBC 9.12.2.2). A wall built to a warm-climate detail, backfilled with site clay and no drain, will move. The other local factor is the freeze count: Ottawa cycles through freezing repeatedly each winter, and every cycle works a poorly drained wall further out of line.

Who should do the work. Landscaping is not licensed in Ontario and only the title Landscape Architect is protected, to full OALA members (OALA on the title). For a wall, two other things matter more: an engineer's stamped design where the height requires one, and a contractor who will state the base depth, the drainage detail and the geogrid schedule in writing. Check WSIB coverage and liability insurance limits too, because a wall failure is a property damage claim rather than a warranty claim.