Retaining Walls in Sydney: What to Know Before You Build One

Sydney is a city built on slopes. Sandstone ridges, benched house pads and cut and fill subdivisions mean a very large number of properties are holding back soil somewhere, whether that is a 400mm garden edge or a 2.4 metre wall across the back of the block. Retaining walls are also the most common piece of structural landscaping that gets built badly, because from the outside a good one and a bad one look identical for the first few years.

Water, not soil, is what pushes a wall over

The instinct is to assume a retaining wall fails because the soil behind it is too heavy. Most of the time it fails because water got trapped behind it. Saturated soil is far heavier than dry soil, and water that cannot drain builds hydrostatic pressure directly against the back of the wall.

A wall built properly has a drainage layer behind it, meaning free draining aggregate rather than the excavated clay going straight back in, a subsoil drain at the base running to a legal discharge point, and a geotextile fabric separating the aggregate from the soil so it does not silt up over time. In blockwork you will usually also see weep holes.

If you are comparing quotes and one is noticeably cheaper, drainage is very often where the difference is. It is completely invisible once the wall is finished, which is exactly why it gets left out.

Height changes the rules, not just the price

In NSW, low retaining walls can often be built as exempt development, while anything above a modest height generally needs council approval and engineering. The threshold most commonly cited for exempt work is 600mm, along with conditions about setbacks from boundaries, not redirecting surface water onto a neighbour, and the land not falling into certain categories.

Above that, expect to need a structural engineer's design and either a development application or a complying development certificate, and expect the wall itself to be built differently, with deeper footings, reinforcement, and in blockwork, core filling and starter bars.

These rules vary with the council, the zoning and the site, so treat the figure above as a starting point and confirm your own situation with your local council before committing to anything.

Surcharge: what sits behind the wall matters

An engineer will ask what loads sit on the ground the wall is holding back. A lawn is one thing. A driveway, a parking space, a pool, a shed or the corner of a building is another entirely, and so is a second wall stepped above the first.

This is the most common reason a wall that looks over specified actually is not. If there is any chance a vehicle will drive or park on the retained side, now or in five years, say so before the design is done rather than after.

Materials, and what each is actually good at

• Treated pine sleepers with galvanised steel posts. The cheapest option and perfectly good for low garden terracing. Ask what treatment level is being used, because timber in permanent ground contact needs a high treatment rating, and even then it has a finite life.

• Hardwood sleepers. Heavier, longer lived than pine and a warmer look, though both the cost and the variation between individual sleepers are higher.

• Concrete sleepers in galvanised or powder coated steel posts. Now the default for a lot of Sydney residential walls. They will not rot or burn, they handle height well with the right post spacing and embedment, and the finish options have improved a great deal.

• Core filled concrete block. A structural wall on a reinforced footing, rendered or left raw. Strong, permanent and the usual choice for higher walls, but it is a bricklaying job on a proper footing and it is priced accordingly.

• Sandstone blocks or boulders. Very much at home in Sydney, particularly where there is existing sandstone on site. Heavy, needs machine access, and the price reflects both the material and the plant required.

• Gabion baskets. Rock filled cages, free draining by their nature, and increasingly used where the wall is meant to be a feature.

Access decides more of the cost than people expect

The difference between a site where an excavator or bobcat can reach the wall line and one where everything is barrowed through a side gate is enormous, and it applies twice: once for excavation and spoil removal, and again for materials coming in. On a tight Sydney block the spoil alone can be several truckloads.

When comparing quotes, check whether excavation, spoil removal and material delivery are all included or listed as provisional. That is where quotes most often differ without appearing to.

Where the retaining wall meets the fence

A lot of retaining walls exist because somebody wanted a fence. Three points are worth knowing.

First, sequence. Where ground levels differ, the wall normally has to be built and backfilled before the fence goes on top of or behind it. Building the fence first and retaining afterwards means doing the fence twice.

Second, height. Fence height is measured from finished ground level, so a 1.8 metre fence sitting on top of a 900mm wall reads as 2.7 metres from the low side. That matters for council height controls and it matters a great deal for pool barrier compliance, where the non climbable zone and effective height are measured from the outside.

Third, responsibility. The Dividing Fences Act 1991 sets out how the cost of a dividing fence is generally shared between neighbours in NSW. A retaining wall is not a dividing fence and is generally not treated the same way, with responsibility usually following whose land is being retained and what created the level difference in the first place. This gets contested often enough that it is worth getting specific advice rather than assuming the cost is automatically split.

Signs an existing wall is in trouble

• Leaning or bowing outward at the top, or a line of sleepers pushing forward

• Stair step cracking through blockwork or render

• Soil or mulch washing out from behind or through the wall after heavy rain

• Timber that is soft or hollow at ground line, which is where rot always starts

• No visible weep holes or drainage outlet anywhere along the base

• Ground behind the wall staying wet long after the rain has stopped

Any of these is a reason to have the wall looked at rather than watched. Retaining walls do not usually fail gradually at the end. They hold, and then they do not.

What a good retaining wall quote tells you

A quote worth comparing should state the finished height and length, the material and its specification, what the footing is, what drainage is included and where it discharges to, whether engineering and approval are included or left to you, and what happens to the spoil.

A Cut Above Landscaping & Carpentry holds NSW Contractor Licence 484384C in structural landscaping, the licence category that covers retaining wall work, and we build walls and fencing across the Sutherland Shire, St George, Canterbury-Bankstown, the Inner West and our other Sydney service areas.

If you have a slope, a wall that is starting to move, or a fence that cannot go in until the levels are sorted out, get in touch and we will come and look at it.

Also worth reading: When Should You Replace a Fence Instead of Repairing It? and Fencing in the Inner West.

Also worth reading: a retaining wall is often the step that makes a usable outdoor space possible in the first place. Decking in Sydney covers what usually goes on top of it, and why the substructure matters more than the boards.