Retaining Walls in London, Ontario: Permits, Design and Drainage
In London, Ontario, a retaining wall under 1 m (about 3.3 ft) high doesn't need a building permit. A wall 1 m or higher does. If a wall is over 1 m in exposed height and sits next to public property, a building entrance or a property the public can access, the Ontario Building Code treats it as a designated structure that must be designed by a qualified professional. Near creeks, valleys or wetlands, you may also need a conservation authority permit. Whatever the height, good drainage behind the wall is what keeps it standing.
Here's what London-area homeowners should know before building or replacing a retaining wall.
When you need a building permit in London
The City of London's building permit page lists projects that don't need a permit. It includes retaining walls less than one metre high. That means walls 1 m or taller need a building permit before construction.
The same page lists a few related thresholds that often come up on the same project:
Decks more than 0.61 m (2 ft) above grade need a permit.
Sheds over 15 m², or with plumbing, need a permit.
Fences don't need a building permit, though pool fences have their own rules.
Measure wall height from the finished grade at the bottom to the top of the wall. Terraced walls built close together can act like one tall wall, so ask the City how they'll be measured before you split one big wall into two smaller ones.
When the Building Code requires a design professional
The Ontario Building Code lists certain structures as "designated structures." These include a retaining wall exceeding 1,000 mm in exposed height next to:
public property,
access to a building, or
private property to which the public is admitted.
Municipal guides, like the Town of Erin's retaining wall guide, explain that these walls need a permit and drawings prepared by a competent design professional. Guards may also be needed where people can walk along the top of the wall.
Even for a residential wall that isn't technically a designated structure, we recommend engineering when:
the wall is tall or holds back a driveway, parking area or structure,
the soil is wet, soft or heavy clay,
the wall is close to a property line or a neighbour's building, or
a previous wall in the same spot failed.
Conservation authority permits
Much of southwestern Ontario falls within regulated areas controlled by conservation authorities. Since April 1, 2024, Ontario Regulation 41/24 sets the rules. According to the Upper Thames River Conservation Authority (UTRCA), regulated areas include:
hazardous lands,
wetlands and land within 30 m of a wetland,
river or stream valleys, and
Great Lakes and inland lake shorelines.
In a regulated area, development, which includes construction, site grading and placing or removing fill, needs a permit. Retaining walls on ravine lots, creek-side yards and shoreline properties often fall into this category.
Which authority you call depends on where you live:
London, St. Marys, Ingersoll: Upper Thames River Conservation Authority (UTRCA)
St. Thomas and central Elgin: Kettle Creek Conservation Authority (KCCA)
Grand Bend, Bayfield, Goderich: Ausable Bayfield or Maitland Valley conservation authorities, depending on location
What makes a retaining wall last
Most retaining wall failures we see in London come down to water, frost and the base. A well-built wall includes:
Excavation to firm subgrade. Topsoil and soft material come out.
A compacted granular base sized for the wall height and block type, placed in thin lifts and compacted.
A buried first course. Part of the bottom row sits below grade to resist sliding.
Clear stone drainage behind the wall, so water can drain instead of building pressure.
A perforated drain pipe at the base, with somewhere for the water to go.
Filter fabric to keep clay from clogging the drainage stone.
Geogrid reinforcement for taller walls, per the manufacturer or engineer.
Proper setback between courses, per the block system.
Grading above the wall that sends surface water away from the top.
Why clay soil makes drainage critical
London's heavy clay holds water and expands when it freezes. Behind a wall, that means hydrostatic pressure in spring and frost heave in winter. A wall with no drainage stone or pipe may look fine for a few years, then start leaning, bulging or cracking. Drainage isn't optional here.
Choosing wall materials
The right material depends on height, style and budget:
Segmental concrete block is the most common choice for residential walls. Systems interlock, have a built-in setback and work with geogrid for taller walls.
Natural armour stone suits a rugged, natural look on slopes and larger walls, but needs heavy equipment to place.
Poured concrete is used for structural walls, often with engineering.
Timber is cheap upfront but has the shortest life in our wet, freeze-thaw climate.
Repair or replace? A quick checklist
Signs your wall needs attention:
Leaning outward or bulging in the middle.
Blocks or stones cracked, shifted or popped out.
Water seeping through joints or pooling at the base.
Soil washing out from under or behind the wall.
A gap opening between the wall and the soil above.
Small, localized problems can sometimes be fixed. Widespread leaning usually means the wall has to be rebuilt with proper drainage and base. Timber walls, common in older London backyards, eventually rot and are usually best replaced with stone or concrete block.
Planning in fall for spring construction
Late fall and winter are ideal for planning:
Book a site visit while you can still see grades and drainage clearly.
Get engineering and permits lined up, which can take weeks.
Check for conservation authority regulation on your lot.
Book construction for spring, once the ground has thawed and dried.
If the ground isn't frozen, smaller walls can sometimes still be built in October and November. Before any digging, locates are required. Our excavation checklist explains how.
Retaining walls often go hand in hand with grading and drainage. If you're dealing with water problems, our lot grading guide is a good next read. For the stone and aggregates behind the wall, see our gravel and aggregate page. For more finished hardscape projects, browse our photo gallery.
FAQ
Do I need a permit for a retaining wall in London, Ontario?
According to the City of London's building permit guidance, a retaining wall less than one metre high doesn't need a building permit. Walls 1 m or higher need one. Always confirm with the City's Building Division before you build.
When does a retaining wall need an engineer in Ontario?
The Ontario Building Code treats a retaining wall over 1,000 mm in exposed height as a designated structure when it's next to public property, access to a building, or private property open to the public. Those walls need a permit and design by a qualified professional. Many contractors also recommend engineering for tall or heavily loaded walls even when it isn't strictly required.
Do I need a conservation authority permit for a retaining wall?
Possibly. If your property is in a regulated area, such as near a river, stream, valley, wetland or shoreline, grading and construction can need a permit under Ontario Regulation 41/24. In London that's usually the Upper Thames River Conservation Authority.
Why do retaining walls fail?
The most common causes are poor drainage behind the wall, a weak or shallow base, no geogrid on taller walls, and too much weight above the wall. Water pressure and frost in clay soils are the biggest enemies in southwestern Ontario.
Can retaining walls be built in the fall?
Yes, as long as the ground isn't frozen and the base can be properly compacted. Fall is also a good time to plan and design walls for spring construction.
Talk to SPM Landscape & Excavation
Planning a wall for next season? Fall and winter are a great time to measure, design and book, so you're first in line when the ground thaws. SPM has served London and southwestern Ontario since 2006, is WSIB covered and carries $1M in liability insurance. Call (519) 474-0008, email spmlandscape@gmail.com, or visit us at 791 Manchester Rd, London, ON.
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