Retaining walls on residential sites: approvals and engineering

A retaining wall is often treated as a landscaping job. It gets a delivery of blocks, a trench dug along the line the client has marked, and a wall built to the height that looks right. The trouble is that the wall is holding back a mass of soil that pushes sideways all the time, and that push increases with the height of the retained ground and with the water it carries. A wall that is built as if it were a garden feature can fail long after the crew has gone.

This article covers the approvals and engineering that residential retaining work may need, the drainage and surcharge issues behind most failures, and the boundary and neighbour considerations that often come up once the work is under way. It does not quote wall heights or thresholds, because those depend on the site and the planning rules that apply to it. It does not cover the choice of materials in detail, which is a separate subject.

The organising idea is that a retaining wall is a structure, and the approval and engineering questions follow from what it is holding back, what is loading the top, and what sits on the ground behind it.

What a retaining wall is actually doing

A retaining wall resists the sideways push of the ground it holds. That push depends on the type of soil, how far it extends above the base of the wall, whether it is dry or saturated, and whether it carries any load on the surface. Once the soil starts to move, the wall has to hold a pressure it was not designed for, and the wall moves with it. The movement is usually gradual at first, which is why the early signs are easy to miss.

The base of the wall also has to hold. The footing carries the weight of the wall and resists the tendency of the wall to tip or slide. A footing that is too small, poorly founded or built on ground that is not what the design assumed will let the wall move even if the wall itself is well built. Most builders can see the wall. The footing and the ground under it are harder to check, and they are where a poor design or a poor build usually hides.

Water is the usual reason walls fail

Water is the single most common cause of retaining wall failure, and it is the one most often under-designed. Water that builds up behind a wall adds its own pressure, and it does so in addition to the soil. A wall with no drainage behind it can hold back far more pressure than it was built to resist, and the water that cannot escape often finds its way out through the joints or the base instead.

Good drainage has a few simple elements. Free-draining material sits directly behind the wall, so water runs down rather than settling in the soil. A drain at the base carries the water to an outlet that discharges it safely away from the wall and the footings. Weep holes or equivalent openings allow water through the wall itself. Filter fabric keeps fine soil from clogging the drain. Each of these is a detail that is easy to skip and expensive to fix once the wall is built, because the wall has to come down to reach them.

The materials and the detail of the drain are covered in the article on retaining wall materials and drainage for residential sites. This article is about why the drain matters to the approval and the engineering. That is the question a builder has to answer before the materials are ordered, not after the blocks are on the site.

The water that reaches the wall also comes from elsewhere. Roof runoff, paving that falls toward the wall, and a gutter or downpipe that discharges behind the wall all add water to the ground the wall is holding. The retaining wall is part of the site drainage, and its design has to take account of where all the water goes.

Surcharge: the loads the wall does not see on the drawing

A surcharge is any load on the ground behind the wall, above or near the top of it. It may be the weight of a driveway that carries vehicles, a pool, a shed, a stack of materials, a paved terrace or the footings of a neighbouring building. Each of these adds pressure to the soil the wall is holding, and each can be forgotten when the wall is designed for the ground alone.

The question to ask before the design is fixed is what sits on the ground behind the wall now, and what will sit there later. A wall built beside a driveway that is later used for a heavier vehicle, or beside a shed that is later extended, can be overloaded by a change that was never designed for. Record the surcharge loads on the design and make sure the engineer knows about them.

When the work needs approval

Whether a retaining wall needs approval depends on the site and on the planning rules that apply to it. The category of the work, the position of the wall relative to the boundary, the height of the retained ground, and the planning controls for the land all feed into the answer. Some walls fall within exempt development, some within complying development, and some need a development application. The thresholds are set by the planning rules and they change, so check them for the site rather than working from memory or from what a neighbour did.

The question of which works can be carried out without an approval is covered in the article on what builders can skip under exempt development in NSW. It is worth reading before deciding that a wall needs no paperwork at all, because the exemption depends on conditions that are easy to overlook on a sloping block.

The NSW Planning Portal explains how planning approvals and the BASIX scheme relate to residential work, and it is the place to start when the question is whether a particular structure needs a formal approval. Where the wall is part of a larger project, the approval for the whole project may cover it, and the retaining work should be read against that approval rather than treated as a separate job. A wall built without the approval it needed can be ordered down, and the cost of that is far higher than the cost of checking first.

When an engineer must design the wall

Approval and engineering are separate questions. An approval may be needed even where the wall is simple, and an engineer may be needed even where no formal approval is required. The engineer’s job is to design the wall for the forces on it and to confirm the footing for the ground it sits on. That is the step that most often gets skipped on residential work, and it is the one that matters most when the wall is holding back a lot of soil.

Engage an engineer where the retained ground is substantial, where the wall carries a surcharge from a driveway, a structure or a pool, where the wall sits close to a boundary or a neighbour’s footing, where the ground is poor or uncertain, or where the wall is stepped in tiers that interact with each other. Where the ground is uncertain, the engineer may ask for a test of the soil before the design is finished. Do not start digging on the strength of an assumption about the ground.

Give the engineer a complete brief. That means the survey of the ground levels, the position of the boundary, the location of any existing footings or services, the loads on the surface behind the wall and the drainage arrangement you intend to use. A design prepared from a partial brief will be a design for a different wall from the one that is built, and the difference usually appears where the information was missing.

The boundary and the neighbour

A retaining wall on or near a boundary raises questions that go beyond the wall itself. The wall may cross the line, its footing may sit under the neighbour’s land, or it may hold back the neighbour’s ground. The obligations that apply in those cases come from the land, the dividing fence arrangements and the planning approval, and they should be checked before the work starts rather than after the neighbour raises the matter.

Talk to the neighbour early, in writing, and describe what is proposed, where the wall will sit and how it will be drained. An informal agreement that is not recorded is hard to prove later, and a neighbour who was not told is likely to find out when the excavation starts. Keep a note of the conversation and of any consent given. Where the wall affects the neighbour’s ground, a survey may be needed to show where the boundary really is.

Working safely on an excavation and at the top of the wall

Retaining work carries two hazards that are often underestimated. The first is the excavation itself. Soil that has been cut back can collapse onto a person in the trench, and the risk is greatest after rain. The second is the fall from the top of the wall or the edge of the cut, which is a working at heights hazard as much as a trench one. Safe Work Australia’s guidance on working at heights sets out the general duties, and the same logic applies to the edges of a retaining excavation.

Keep spoil and materials back from the edge of the cut, and do not let plant run close to it. Support or batter the excavation as the design and the ground require. Get the trench inspected before the footing is poured, because a footing poured into a disturbed trench is a defect that cannot easily be seen once the wall is up.

Plan the excavation around the weather. Cut-back ground that has been left open through a wet week is the most dangerous state a retaining cut can be in, and it is also the state in which water finds its way into the soil the wall is meant to hold. Where rain is forecast, cover the cut, keep water off the top of it, and do not open a longer length than can be finished and protected before the forecast arrives. The cost of covering a cut is small. The cost of a collapse or a saturated trench is measured in injury, delay and the work that has to be redone.

Worked example: a wall that was built before the drain

Consider a builder in the Hunter Valley who is asked to build a retaining wall to hold back the rear of a sloping block. The client has marked the line and the height they want, and the builder has built walls like it before. He does not check the planning position, and he does not ask for an engineer’s design because the wall looks modest. The footing is poured into a trench that was dug before the rain, and the drainage behind the wall is planned to go in after the blocks are laid.

The rain arrives before the drainage is in place. The water that runs off the rear of the block and off the paved area above the wall has nowhere to go but into the soil behind the blocks. Within a few weeks the wall has moved, a joint has opened, and the neighbour has noticed the change on their side of the boundary. The builder has to take down part of the wall, install the drain and rebuild it on a footing that is checked. The owner has a dispute with the neighbour, a planning question to answer and a wall that has cost much more than it would have cost to do properly. The lesson is that the drainage and the approval were the cheap parts of the job, and they were the parts that were skipped.

Records to keep for a retaining wall

Keep the approval, the engineer’s drawings and the notes of any inspection in the job file. Photograph the trench before the footing is poured, the drain as it is laid, the filter material before backfill, and the outlet once it is connected. These records are the evidence that the wall was built to the design, and they are what you will need if the wall is questioned years later.

The wider set of licence obligations and the continuing learning that goes with them are set out in the PointCPD guide to NSW builder CPD. Structural and drainage work is an area where the technical standard is the builder’s responsibility, and a short course in the design and the drainage of retaining walls is worth considering if the work is a regular part of the business.

Questions builders ask

• Approvals
• NSW Compliance
• Structure

Published 17 October 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.