Acid Sulfate Soils in NSW: What They Are and How They Affect Your Approval

If your property is anywhere near the coast or a low-lying area, there's a chance it's mapped for acid sulfate soils. It sounds alarming, but it's a manageable constraint once you understand it. Here's a plain-English guide to what acid sulfate soils are, how to tell if your land is affected, and what it means for whether you can use the fast-track CDC pathway or need a full DA.

What are acid sulfate soils?

Acid sulfate soils are naturally occurring soils that contain iron sulfides. Left alone and waterlogged, they're completely harmless. The problem starts when they're dug up or drained and exposed to air, because they can then react to produce sulfuric acid. That acid can damage building foundations, pipes and concrete, and can leach into nearby drains, creeks and wetlands, harming the environment. They're most common in low-lying land near the coast, generally less than five metres above sea level.

The important thing to know is that they're a well-understood constraint. They don't stop you building. They just need to be recognised and managed properly.

How do I know if my land is affected?

Acid sulfate soils are mapped, and each affected area is given a class from 1 to 5. You can check whether your property is mapped, and what class it is, on the NSW Planning Portal Spatial Viewer, or it will show up on a planning certificate for the property. The class tells you how close to the surface the soils are likely to be found.

The five classes, in plain terms:

Class 1 — soils likely to be found on and below the natural ground surface.
Class 2 — soils likely to be found below the natural ground surface.
Class 3 — soils likely to be found more than 1 metre below the natural ground surface.
Class 4 — soils likely to be found more than 2 metres below the natural ground surface.
Class 5 — soils not typically present, but the land sits within 500 metres of Class 1, 2, 3 or 4 land, so works that lower the watertable could still have an impact.

In short, the lower the class number, the closer the soils are to the surface, and the more likely your works are to disturb them.

When does the class actually matter?

This is the part people miss. It's not simply having a class on your land that triggers a requirement, it's whether your works will disturb the soil to the relevant depth or lower the watertable. Broadly, the more sensitive the class, the shallower the works that trigger the need for consent. On Class 1 land, any works can trigger it. On Class 2, it's works below the ground surface or that lower the watertable. On Classes 3 and 4, it's works beyond 1 metre and 2 metres deep respectively, or that lower the watertable by those depths. Class 5 is about protecting nearby lower-class land from a lowered watertable.

There are also some relief valves. Small-scale works that disturb only a very minor amount of soil and won't lower the watertable generally don't trigger it. And where a preliminary soil assessment by a suitably qualified consultant shows that a management plan isn't needed, and the council confirms that in writing, consent may not be required either.

Which classes can use the CDC (fast-track) pathway?

This is where the class really counts. As a general rule, land mapped as Class 1 or Class 2 cannot use the complying development (CDC) pathway. If your property is Class 1 or 2, a CDC is off the table and your development will need to go through a Development Application instead.

Land mapped as Class 3, Class 4 or Class 5 can generally still use the CDC pathway, provided the development also meets all the other complying development standards, including the separate limits on how deep you can excavate and how much fill you can bring in.

One useful nuance: if only part of your lot is mapped as Class 1 or 2, complying development can still be carried out on the part of the lot that isn't affected. So the mapping doesn't always rule out a CDC for the whole property.

Which classes go through a DA?

Any class can be dealt with through a Development Application. In practice, a DA is the pathway when your land is Class 1 or Class 2, or when your works trigger the need for consent and the CDC pathway isn't available for another reason. Where consent is required, the council will generally want to see an acid sulfate soils management plan prepared by a suitably qualified consultant, setting out how the soils will be handled so they don't cause harm.

What you'll likely need

If acid sulfate soils are in play, the usual first step is a preliminary assessment by a qualified environmental or geotechnical consultant. That assessment either clears the works or leads to a management plan that sets out how the soil will be treated, neutralised or contained during construction. It's a routine part of building in coastal NSW, and it's very manageable with the right consultant on board.

Where this leaves you

Acid sulfate soils sound scarier than they are. For most homeowners the practical takeaway is simple: if your land is Class 1 or 2, expect a DA rather than a CDC, and budget for a soil assessment. If it's Class 3, 4 or 5, you may still be able to fast-track, depending on how deep your works go.

If you're not sure whether your property is affected, or what pathway it puts you on, we can check the mapping and let you know exactly where you stand before you commit to a design.