Whether you need approval generally depends on how tall the wall is, whether there's any load on the ground behind it, and how close it sits to buildings, boundaries and other walls. A low wall around a garden bed might not need a building permit. Put a driveway along the top of that same wall, though, and it's a different story, because the vehicles add pressure the wall has to be engineered to resist.
In the Northern Territory, building work is governed by the Building Act 1993 and the Building Regulations. Building permits come from registered building certifiers working in private practice, and land use and development are dealt with separately under the NT Planning Scheme. Builders doing permit work usually need to be registered with the NT Building Practitioners Board. The rules have conditions attached and are updated over time, so use this guide as a general starting point to gauge whether your project is likely to need a permit. For a firm answer on your own wall, speak to a registered building certifier.
When a permit is likely
If any of the following sounds like your wall, plan on talking to a certifier and an engineer, and you may well need a building permit.
- It's bigger than a low garden wall: height is generally measured including the soil behind it, so your wall may count as taller than it looks.
- There's a load above it: such as a driveway, house, pool, carport, shed or built-up fill pad.
- It's near a building, a boundary or another retaining wall, or it forms part of a pool fence.
- Your land has a planning or hazard issue, such as flooding, storm surge or a drainage easement, or the wall would push stormwater onto your neighbour's block.
These factors often overlap, so a wall that looks obviously exempt can end up needing approval. Terraced walls catch a lot of people out: two short walls built close together may be assessed as one tall wall, however low each one is.
When you may not need one
The minor-work exemptions are designed for small, simple walls.
- Short walls retaining only garden soil, with no load in the area that bears on the wall.
- A good distance from buildings, boundaries and other retaining walls, and not part of a pool fence.
- In line with planning and council rules: even exempt work may need to comply with the NT Planning Scheme, easements and council stormwater requirements.
Exempt doesn't mean it can be built any old way. Nobody will be checking the work, so quality workmanship matters even more.
What is surcharge?
This is the term we're asked about most often, and it's usually the reason a straightforward wall ends up needing an engineer.
Put simply, surcharge is any load on the ground behind your wall other than the soil itself. In Darwin it's most often a driveway. The land here is largely flat, but level changes are everywhere: a carport or house pad built up on fill in Zuccoli or Johnston, a driveway next to a raised garden bed in the northern suburbs, or a cliff-top property in Nightcliff or Fannie Bay. Vehicles are heavy and always coming and going, and that load presses down into the soil behind the wall. Walls with surcharge generally need an engineer's design to AS 4678, and our retaining walls crew builds to that design.
A quick guide to common situations
Use this table as a first check, then confirm the specifics with a registered building certifier.
| Your situation | Will you probably need a permit? |
|---|---|
| Higher than a low garden wall | Probably, with an engineer and building certifier involved |
| Any height, with a driveway, structure or fill pad behind it | Probably, with an engineer and building certifier involved |
| Near a building, boundary or another wall, or part of a pool fence | Probably; check with a certifier |
| Land subject to flooding, storm surge or an easement | Possibly; check with a certifier and your local council |
| Part of a bigger job that already has a building permit | May already be covered; ask your certifier |
| Low wall, garden soil only, no surcharge, well away from buildings and walls | May be exempt; check every condition with a certifier |
What skipping a permit can cost you
The consequences usually arrive years down the track, which is why skipping the permit can seem tempting and why it's a bad idea. A building and pest inspector or a buyer's conveyancer may flag the wall, and getting it certified retrospectively can mean uncovering completed work or rebuilding it entirely. That's not something you want to deal with while you're trying to settle a sale.
Get the drainage right, whatever the paperwork
If you remember one thing from this guide, make it this. Most retaining walls fail because of water, and a Darwin wet season, particularly on clayey or slow-draining soil, will expose any shortcut.
How we build walls that last
Saturated soil weighs a great deal more than dry soil and pushes on the wall with hydrostatic pressure it wasn't built for. With nowhere to drain, that pressure builds until the wall leans, bulges or cracks, and a monsoon downpour can soak the ground in a few hours. Weep holes, an ag pipe along the base, geotextile fabric and clean drainage gravel behind the wall stop that happening, and they're cheap compared with a rebuild. We put them into every wall we build, whether it's inspected or not, and we work with your engineer and certifier when a permit is needed. Before any digging, our excavation team also lodges a Before You Dig Australia request to locate underground services.