A sloping site does not automatically mean a slope stability report is required for a development application. The slope stability assessment (for DA requirements) depends on the site's characteristics, the proposed works, the applicable planning controls and, in some cases, council requirements.
The question becomes important when a proposal changes the ground profile, adds loads near a slope, involves excavation or filling, or sits within an identified geotechnical hazard area. Knowing when to investigate slope stability can help developers address geotechnical constraints before the DA is finalised.
A Slope Does Not Automatically Mean You Need a Report
The presence of a slope alone is not enough to conclude that a slope stability report is mandatory.
What matters is whether the existing ground conditions and proposed works create a geotechnical issue that needs to be assessed under the applicable planning framework.
A relatively simple development may have limited implications for an existing slope. A project involving substantial excavation, retaining walls or construction close to a slope crest can raise very different considerations.
This is why when to get a slope stability assessment should be considered in relation to both the site and the proposed development.
When Slope Stability Becomes Relevant to a DA
Certain site and development characteristics can make slope stability relevant to the development assessment. These include:
- Development on or near a steep slope
- Significant cut or fill
- Retaining walls or substantial earthworks
- Basement excavation
- Building loads close to a slope crest
- Development near the toe of a slope
- Existing signs of ground movement
- Sites identified as having landslide or geotechnical hazards
- Changes to drainage or groundwater conditions that could affect slope behaviour
- Subdivision or larger development involving substantial changes to site levels
These conditions do not mean that every project automatically requires a statutory slope stability report.
They indicate circumstances where slope stability may need to be assessed and where council or the relevant authority may require supporting geotechnical information.
The appropriate scope depends on the planning controls, site conditions and proposed development.
NSW: How Slope Stability Fits Into the DA Process
NSW does not have one universal rule requiring every development on sloping land to include a slope stability report.
The NSW Planning Portal notes that site assessment requirements depend on the particular site and build. Geotechnical reports can form part of the supporting information for development where subsurface conditions need to be understood.
Local planning controls can be more specific.
Lake Macquarie, for example, identifies development types that do not require a slope stability assessment and specifies other development where a geotechnical report must accompany the application. It also allows Council to request a geotechnical report after lodgement in certain circumstances.
This gives developers an important practical rule:
There is no "one slope = one report" requirement for every NSW DA. Check the controls applying to the actual site.
Geotechnical requirements can also apply to areas affected by known slope hazards. The appropriate assessment therefore needs to be determined from the planning controls and the characteristics of the proposed development, rather than from slope alone.
South Australia: What Developers Need to Check
South Australia uses a different planning framework.
The Planning and Design Code is the statutory planning instrument used to assess development applications across the state. Its policies are applied according to the property and proposed development, and the online Code can be searched to identify the provisions and assessment pathway that apply.
For a sloping site, developers should:
- Identify the Planning and Design Code provisions applying to the property.
- Check the site's planning constraints and assessment pathway.
- Determine whether slope, landslide or other geotechnical considerations require supporting information.
- Scope the geotechnical investigation around the proposed development.
The important point is that NSW requirements should not simply be carried across to South Australia. The applicable SA planning provisions need to be checked for the specific site and development.
What a Slope Stability Report Needs to Establish
Once slope stability becomes relevant, the next question is what the assessment actually needs to establish.
Depending on the project, a slope stability report may consider:
- Existing ground profile and slope geometry
- Soil and rock conditions
- Groundwater where relevant
- Existing indicators of instability
- Proposed cuts, fills and structural loads
- Potential failure mechanisms
- Stability under relevant conditions
- Effects on the proposed development and nearby structures
- Geotechnical recommendations for design and construction
The investigation may combine site inspection, geotechnical investigation, subsurface investigation, laboratory testing and engineering analysis depending on the complexity of the site.
The report is not simply a statement that a slope is "stable" or "unstable". It assesses the interaction between existing ground conditions and the proposed development.
That distinction matters because the same slope can require different considerations depending on what is being built, where it is located and how the development changes the ground.
Why the Timing Matters
A developer should ideally identify slope stability requirements before the development design is locked in.
If the assessment identifies constraints, the design may need to account for:
- Revised building location
- Reduced excavation
- Different cut and fill geometry
- Retaining requirements
- Drainage measures
- Foundation considerations
- Construction sequencing
The value of early assessment is therefore not limited to satisfying a DA requirement. Geotechnical findings can influence the development itself.
Identifying those constraints after the design has been finalised can leave fewer options for responding to them.
Do You Need a Landslide Risk Assessment or a Slope Stability Report?
A slope stability assessment and a landslide risk assessment can address related issues, but they are not necessarily interchangeable.
A slope stability assessment focuses on whether the ground or slope has adequate stability under the conditions being assessed.
A landslide risk assessment considers the likelihood and consequences associated with a landslide hazard and can involve a broader risk framework.
The appropriate scope depends on the site, development and applicable planning requirements.
So, a sloping site does not automatically mean that a full landslide risk assessment DA is required. The first step is to establish what geotechnical question needs to be answered for the proposed development.
When to Get the Assessment Before Lodging Your DA
Consider obtaining geotechnical advice before finalising the DA where:
- The site is significantly sloping
- Substantial excavation or filling is proposed
- A basement or retaining structure is planned
- The development is close to a slope crest or toe
- There are signs of previous movement
- The site is mapped or identified as having a geotechnical hazard
- The council or applicable planning controls indicate that geotechnical documentation is required
The earlier question to answer is:
Could slope stability affect the development we are currently designing?
If the answer may be yes, understanding the ground conditions before the DA is finalised gives the project team more opportunity to respond through the design.
What BHM's Slope Stability Assessment Can Support
BHM Geotechnical's slope stability and landslide investigations can support developers where ground conditions need to be understood before or as part of development planning.
Depending on the project, the assessment can support:
- Assessing existing slope conditions
- Evaluating the influence of proposed development
- Identifying potential instability mechanisms
- Providing geotechnical parameters and recommendations
- Informing excavation, retaining and foundation decisions
- Supporting development application documentation where required
The scope should be based on the site's conditions and proposed works rather than a generic checklist for every hillside development.
Check the Geotechnical Requirements Before You Lodge
Before lodging a DA on a sloping site, establish whether slope stability assessment is relevant to the proposed works and what level of geotechnical information is required.
Addressing the question early allows the investigation to reflect the actual development and gives the design team an opportunity to respond to geotechnical constraints before the project progresses too far.
Frequently Asked Questions
Is a slope stability report always required for a DA in NSW?
No. Requirements depend on the site, proposed development, applicable planning controls and, in some cases, council-specific requirements. Some NSW councils specify development types where a geotechnical report is required.
When should I get a slope stability assessment?
Ideally, before the development design is finalised where the site or proposed works could affect slope stability.
Does a sloping block automatically require a geotechnical report?
Not necessarily. The degree of slope, site conditions, proposed works and applicable planning requirements all need to be considered.
Is a slope stability report the same as a landslide risk assessment?
Not necessarily. They can address related issues but may have different scopes depending on the site, development and planning requirements.
Can council request a geotechnical report after a DA is lodged?
In some circumstances, yes. For example, Lake Macquarie's planning controls allow Council to request a geotechnical report after lodgement for certain development types.
Does South Australia have the same slope stability DA requirements as NSW?
No. NSW and South Australia operate under different planning frameworks. In SA, the applicable Planning and Design Code provisions and assessment pathway need to be checked for the specific property and development.
What triggers a slope stability assessment?
A slope stability assessment may be required when proposed development could affect ground stability. Common triggers include steep slopes, significant cut and fill works, retaining walls, basement excavation, development near slope crests or toes, existing ground movement, or identified geotechnical hazards. The requirement depends on the site conditions, proposed works and applicable planning controls.
