Answer first: A NSW development application needs an acid sulfate soils assessment when the works listed for your lot's class on the LEP Acid Sulfate Soils Map are proposed — from any works on Class 1 land to excavation below set depths, or watertable lowering, on Classes 2–5. A preliminary assessment may clear the site; otherwise council needs a management plan.
By: giantA Pty Ltd · Updated 20 October 2026
What are acid sulfate soils, and why does council care?
Acid sulfate soils are natural sediments that contain iron sulfides. Left undisturbed and waterlogged, they are harmless. Once they are excavated, drained or exposed to air, the sulfides oxidise and produce sulfuric acid. The NSW environment department notes that the acid makes metals such as iron and aluminium more soluble, can be washed into waterways, kills plants and aquatic life, and can corrode concrete, iron, steel and some aluminium alloys.
They occur in every coastal estuary in NSW, typically under low-lying estuarine land, mangroves, saltmarsh, floodplains and backswamps. In Greater Sydney that usually means land near tidal rivers, bays and lagoons — but the reliable starting point for any lot is your council's local environmental plan (LEP) mapping.
On a residential site the risks are twofold: acid leachate from an excavation or dewatering pump can leave the site through stormwater, and acidic soil and groundwater can attack footings, piles, slabs and buried services. That is why councils treat acid sulfate soils as a standalone consent issue.
Where does the acid sulfate soils rule come from in a NSW DA?
Most NSW LEPs made under the Standard Instrument contain an “Acid sulfate soils” clause with the same structure. The clause number differs between councils, so always check your own LEP:
| Local environmental plan | Acid sulfate soils clause |
|---|---|
| Canterbury-Bankstown LEP 2023 | Clause 6.1 |
| Parramatta LEP 2023 | Clause 6.1 |
| Wollongong LEP 2009 | Clause 7.5 |
| Coffs Harbour LEP 2013 | Clause 7.1 |
The clause's objective is that development does not disturb, expose or drain acid sulfate soils and cause environmental damage. It makes development consent required for the works listed against each class of land on the Acid Sulfate Soils Map; says consent must not be granted unless an acid sulfate soils management plan has been prepared in accordance with the Acid Sulfate Soils Manual and provided to the consent authority. and gives an exception where a preliminary assessment shows a management plan is not needed and council confirms that in writing.
Council also weighs acid sulfate soils as part of its general merit assessment under section 4.15 of the Environmental Planning and Assessment Act 1979, because the LEP is a matter it must consider and the likely impacts of the development include impacts on soil, water and structures. Some older LEPs add an agency referral step, so read your full clause.
What do Class 1 to Class 5 mean on the Acid Sulfate Soils Map?
The map shows probability and likely depth, not confirmed presence. The NSW Planning Portal dataset describes Class 1 as likely on and below the natural ground surface, Class 2 below the surface, Class 3 beyond 1 metre, Class 4 beyond 2 metres, and Class 5 as land not typically affected but within 500 metres of Class 1–4 land.
| Class | Works that need consent under the LEP clause | Typical residential examples to check |
|---|---|---|
| 1 | Any works | Any new building, footing, driveway, service trench or landscaping earthworks |
| 2 | Works below the natural ground surface; works by which the watertable is likely to be lowered | Strip and pad footings, piers, piling, in-ground tanks, service trenches, subsoil drainage |
| 3 | Works more than 1 m below the natural ground surface; works likely to lower the watertable more than 1 m below the natural ground surface | Swimming pools, basements, lift pits, deeper on-site detention or rainwater tanks |
| 4 | Works more than 2 m below the natural ground surface; works likely to lower the watertable more than 2 m below the natural ground surface | Basement car parks, deep piling or bulk excavation |
| 5 | Works within 500 m of adjacent Class 1–4 land that is below 5 m AHD and by which the watertable is likely to be lowered below 1 m AHD on that adjacent land | Basements or construction dewatering near mapped low-lying land |
The examples column is our practitioner reading, not part of the clause. Whether a pool or basement triggers it depends on depth below natural ground and groundwater effects — so accurate levels on your detail and level survey matter. Class 5 is most often missed: a basement that draws down groundwater can affect neighbouring mapped land.
The exceptions are narrow. The Standard Instrument wording excludes works that disturb less than 1 tonne of soil and are not likely to lower the watertable, plus certain public authority works. They rarely help a new dwelling, pool or basement.
When is a preliminary assessment enough?
If your works fall within the clause, you have two ways forward:
- Preliminary assessment route. A suitably qualified consultant prepares a preliminary assessment in accordance with the Acid Sulfate Soils Manual. If it shows a management plan is not required, you provide it to council, and council confirms the assessment in writing. Consent is then not required under that clause — the rest of the project still needs its normal approval.
- Management plan route. If acid sulfate soils are present at or above the manual's action criteria, or you choose to assume they are, an acid sulfate soils management plan is prepared and lodged with the DA. Council cannot grant consent under the clause without it.
The manual allows a proponent to accept that acid sulfate soils will be disturbed and go straight to a management plan. That can suit Class 1 or 2 land with obvious estuarine clays; elsewhere, testing often pays for itself because a negative result can remove the need for a plan.
What does an acid sulfate soils assessment actually involve?
The Acid Sulfate Soils Manual 1998, published by the Acid Sulfate Soil Management Advisory Committee (ASSMAC), remains the NSW reference document named in LEP clauses. The state environment department says later national guidance supplements but does not replace it. The manual's assessment guidelines set out a staged process:
- Check the Acid Sulfate Soils Planning Maps. Confirm the class of land for the whole footprint, including driveways, services and stormwater lines.
- Check the geomorphic and site criteria. The manual lists indicators such as recent (Holocene) sediments, soil horizons below 5 m AHD, marine or estuarine sediments, coastal wetlands and backswamps, and vegetation such as mangroves, reeds, rushes, swamp oak, paperbark and swamp mahogany.
- Look at soil and water indicators in the field. This includes field pH and the peroxide test described in the manual's appendix, plus signs such as jarosite (yellow mottling).
- Confirm with laboratory analysis. Samples are analysed for oxidisable sulfur and acidity and compared with the action criteria.
For Class 4 land, or Class 5 land within 500 metres of Class 1–4 land, the manual also recommends a preliminary hydrology study to work out whether the activity is likely to change groundwater levels — current groundwater depth and flow, likely drawdown, and the resulting temporary or permanent change in the watertable.
What are the action criteria?
Table 4.4 of the manual sets the action criteria that trigger the need for a management plan, by soil texture and tonnage disturbed:
| Soil texture (approx. clay content) | 1–1,000 tonnes disturbed: sulfur trail (% S oxidisable) | 1–1,000 tonnes: acid trail (mol H+/tonne) | More than 1,000 tonnes: sulfur trail / acid trail |
|---|---|---|---|
| Coarse — sands to loamy sands (≤5% clay) | 0.03 | 18 | 0.03 / 18 |
| Medium — sandy loams to light clays (5–40%) | 0.06 | 36 | 0.03 / 18 |
| Fine — medium to heavy clays and silty clays (≥40%) | 0.1 | 62 | 0.03 / 18 |
Results at or above these levels require a management plan. Councils may add sampling expectations in a DCP — a Central Coast Council DCP chapter published for exhibition, for example, sets minimum sample numbers, sampling depths below the excavation and AS 4969 laboratory methods — so check your DCP before the consultant prices the job.
What must an acid sulfate soils management plan contain?
The manual is clear that there is no set formula — the level of detail should reflect the risk, the size of the project and the sensitivity of the receiving environment. As a minimum, it says a plan should contain:
- an overview of the environmental attributes of the site and surrounds;
- an overview of the proposed works;
- mitigation strategies, with a construction and operational schedule, for the disturbance itself, excavated soil (storage, treatment or use) and any acid leachate;
- a monitoring programme for soil, surface water and groundwater — parameters, locations, frequency, analyses, laboratory, trigger procedures and reporting;
- a description of any pilot project or field trial where new mitigation methods are proposed; and
- contingency procedures if management fails, including remedial and restoration actions.
On a residential site the plan typically covers stockpiling, containment and neutralisation of spoil (commonly with lime at a rate calculated from lab results), testing and treatment of pumped water before discharge, and record keeping. The manual says the consent authority should require implementation of the plan as a condition of approval — so treat it as a construction document, not just a lodgement document.
Can you use a complying development certificate on acid sulfate soils land?
Not on Class 1 or Class 2 land for most housing codes. Clause 1.19 of the State Environmental Planning Policy (Exempt and Complying Development Codes) 2008 says that to be complying development under the Housing Code, Low Rise Housing Diversity Code, Inland Code, Rural Housing Code, Agritourism and Farm Stay Accommodation Code or Greenfield Housing Code, the development must not be carried out on land identified on an Acid Sulfate Soils Map as Class 1 or Class 2. The Industrial and Business Buildings Code has the same exclusion.
Class 3, 4 and 5 land is not excluded by that clause. However, a pool, basement or dewatering on that land can still engage the LEP acid sulfate soils clause, so confirm the position with your certifier before you commit to a CDC programme. Our guides to CDC vs DA pathways and the DA/CDC checklist explain other land-based exclusions common on low-lying sites, such as flood and foreshore constraints.
How does an acid sulfate soils report fit with your other DA reports?
Coordinating these inputs early avoids paying twice for boreholes and contradictions council will pick up:
- Geotechnical investigation. The same boreholes can often be sampled for acid sulfate soils if the brief says so in advance. See our guide to soil testing and geotechnical investigation.
- Contamination. The environmental consultant may also be preparing a PSI, DSI or RAP; one combined scope is usually more efficient.
- Waste and spoil disposal. The EPA's Waste Classification Guidelines include a separate part on acid sulfate soils, and the department notes they apply where acid sulfate soils are transported and treated offsite. Under the guidelines' definition, virgin excavated natural material must not contain sulfidic ores or soils, so acid sulfate spoil needs its own classification. Flag this in your waste management plan.
- Stormwater and dewatering. Discharge of pumped groundwater interacts with your stormwater and OSD design. Construction dewatering may need separate water approvals — confirm early with your engineer.
- Flood. Low-lying mapped land often carries flood planning controls too.
- Construction management. Stockpiles, bunding and water treatment belong in your construction management plan.
How should you sequence acid sulfate soils in your DA programme?
The cheapest acid sulfate soils outcome is a design that avoids the trigger. We work in this order:
- Confirm the class from the LEP Acid Sulfate Soils Map (available through the NSW Planning Portal spatial viewer and council mapping), and note any adjoining Class 1–4 land for Class 5 sites. Read it alongside your section 10.7 planning certificate and other site constraints.
- Measure the trigger depth. Use survey levels to compare natural ground with proposed excavation for footings, pools, tanks, lift pits and basements.
- Test design options. Raise the pool, reduce the basement, move tanks above ground or change the footing system to limit disturbance and drawdown.
- Brief the consultants together. Combine geotechnical, acid sulfate soils and, where relevant, contamination and groundwater scopes.
- Decide the route. Preliminary assessment with council's written confirmation, or a management plan lodged with the DA.
- Raise it at pre-lodgement. Ask council at a pre-DA meeting what it expects, then address the clause in your Statement of Environmental Effects.
- Carry it through. The builder's programme, construction certificate documents and site set-up must reflect the plan's conditions.
What mistakes cause requests for information on acid sulfate soils?
The requests for further information we see are predictable:
- No reference to the clause at all in the Statement of Environmental Effects on a mapped lot.
- Depth assumptions without levels — a “1 m deep” pool whose excavation, including base and drainage layers, goes deeper.
- Ignoring the watertable limb of the trigger, especially for basements and Class 5 land.
- A geotechnical report that mentions acid sulfate soils in passing but does not follow the manual's assessment steps or compare results with the action criteria.
- No spoil or water disposal pathway, or one that conflicts with the waste management plan.
- Treating council's silence as confirmation. Under the Standard Instrument wording, the preliminary assessment exception relies on council's written confirmation.
Our DA checklist lists the other supporting documents councils commonly ask for on constrained sites.
Frequently asked questions
How do I find out if my property has acid sulfate soils mapping?
Check the Acid Sulfate Soils Map in your council's LEP, through the NSW Planning Portal spatial viewer or council mapping. Your planning certificate and council's DCP may also refer to it. Mapping shows probability, not confirmed presence.
Is an acid sulfate soils report the same as a geotechnical report?
No. A geotechnical report deals with ground conditions for foundations. An acid sulfate soils assessment follows the Acid Sulfate Soils Manual. The same consultant and boreholes can often cover both if briefed early.
Do I need a management plan for a swimming pool?
It depends on the class and depth. On Class 1 land any works are caught; on Class 2 any excavation below natural ground; on Class 3 and 4 only deeper works or watertable lowering. A preliminary assessment may remove the need.
Can I build under a CDC if my land is Class 3?
The Codes SEPP land exclusion in clause 1.19 covers Class 1 and Class 2 land. Class 3 land is not excluded by that clause, but deeper works may still engage the LEP clause — confirm with your certifier.
What happens to excavated acid sulfate soil?
It is managed under the management plan — typically contained, treated (often with lime) and either reused on site or classified and disposed of under the EPA's Waste Classification Guidelines. It cannot be treated as virgin excavated natural material.
Does a preliminary assessment have to be approved by council?
Under the Standard Instrument wording, the exception applies only if the preliminary assessment is given to council and council confirms it by written notice. Check your LEP's clause for the exact wording.
How giantA can help
giantA prepares DA and CDC documentation across Greater Sydney, including level analysis showing whether footings, pools and basements trigger your LEP clause, design options that reduce excavation, and consultant coordination. See our services and architectural services, read our guide to contaminated land and SEPP Resilience and Hazards, or ask giantA via Help.
Preferred sources (verify current versions)
- NSW Environment and Heritage — Acid sulfate soils
- Acid Sulfate Soils Manual 1998 (ASSMAC) — planning, assessment and management guidelines
- NSW EPA — Waste Classification Guidelines Part 1 (2014)
- Canterbury-Bankstown Local Environmental Plan 2023 — clause 6.1 Acid sulfate soils
- Parramatta Local Environmental Plan 2023 — clause 6.1 Acid sulfate soils
- State Environmental Planning Policy (Exempt and Complying Development Codes) 2008 — clause 1.19
- NSW Planning — Exempt and complying development fact sheet
- Coffs Harbour Local Environmental Plan 2013 — clause 7.1 Acid sulfate soils
- Wollongong Local Environmental Plan 2009 — clause 7.5 Acid Sulfate Soils
- NSW Planning Portal — Environmental Planning Instrument: Acid Sulfate Soils dataset
- Environmental Planning and Assessment Act 1979
- Central Coast Council — DCP chapter 3.8 Acid Sulfate Soils (exhibition document)