Landslip and steep-slope construction overlays in AU residential

TL;DR

If your lot sits inside a council landslip overlay, or has a slope steeper than roughly 15-18 degrees (about a 1:3 gradient), you almost certainly need a geotechnical report before lodging a DA. The report must follow AS 1726-2017 for site investigation and classify landslide risk under the AGS 2007 framework on a five-level scale from Very Low to Very High. Most councils accept residential development up to Moderate risk; High and Very High classifications typically require engineered solutions, concurrent drainage referrals, or a prohibition on residential use altogether. The cost jump from a standard flat-lot footing ($10-15k) to an engineered solution on a Moderate or High landslip site ($40-80k+) is the budget shock that catches builders and clients off guard. Find the overlay on the council mapping portal before you design anything.

What this article is for

Tree-covered hills look like great real estate. They often are. But an old landslip scar, a clay-rich geology, or a slope past the council’s trigger threshold can turn a promising lot into a DA that comes back with six referrals and an engineering bill that rewrites the project feasibility.

This article covers:

How landslip overlays work and which council areas are most active.
The slope degree thresholds that trigger a geotechnical assessment.
The AGS 2007 risk classification framework that geotechs use and councils reference.
Structural implications of steep-slope construction.
Cost ranges so you can price accurately at feasibility.
State-by-state variance in how the overlay is administered.

This is not a substitute for a site-specific geotechnical report. It is the orientation you need before you engage a geotechnical engineer.

When the landslip overlay applies

Councils map landslip risk in two ways: slope-based triggers and specific-hazard overlays.

Slope-based triggers apply when the natural ground slope across any part of the building envelope exceeds a set threshold, typically 15 to 18 degrees (roughly 1:3 to 1:3.3). Some councils state this as a percentage (25-33%). Below that threshold, standard residential controls apply. Above it, a geotechnical report is a condition of any DA or development approval.

Specific landslip overlays identify land with known geotechnical hazard regardless of slope: historical slip scars, mapped unstable geology, coastal bluffs, or creek-adjacent cut batters. These overlays appear in the council LEP or DCP (NSW), the planning scheme (VIC/QLD/SA) or the Local Provisions Schedule (TAS). The overlay is the trigger, not just the slope angle. A lot sitting at 12 degrees inside a mapped overlay still requires a geotech report.

Check the council’s online mapping portal before you do anything else. In NSW, the Section 10.7 planning certificate will also disclose a known landslip or slope constraint as a matter affecting the land.

Geotechnical assessment triggers

Trigger	Typical requirement
Slope > 15 degrees (some councils 18 degrees)	Preliminary geotechnical assessment at minimum
Slope > 18 degrees (approximately 1:3)	Full geotechnical report per AS 1726-2017
Land within a mapped landslip overlay	Full report regardless of slope
Cut or fill exceeding 1 m in a slope area	Geotechnical sign-off on earthworks design
Retaining walls over 1 m on a slope	Structural engineering + geotechnical input
Known slip history on or adjacent to the lot	Full report; council may require peer review

The report must be prepared by a suitably qualified geotechnical engineer or engineering geologist. AS 1726-2017 sets the minimum requirements for the site investigation: borehole or test pit program, soil and rock classification, interpretive report with a geotechnical model, and an assessment of project-specific risks. For landslip, the geotechnical engineer then layers on a risk assessment per the AGS 2007 Practice Note.

Some councils (notably Northern Beaches NSW) map all land into geotechnical planning classes (Classes 1-7) based on geology and slope combination. The class determines which assessment level applies, from “no assessment required” up to “detailed geotechnical report and hydrogeological report.”

The AGS 2007 risk framework

The Australian Geomechanics Society Practice Note Guidelines for Landslide Risk Management (2007) is the reference framework used by geotechnical engineers and referenced by most councils across Australia when assessing landslide risk on residential sites.

The framework produces a qualitative risk classification by combining the likelihood of a landslide occurring with the consequences if it does (loss of life, property damage). For residential development, the risk-to-life assessment is the primary output.

Risk level	Annual probability of fatality (indicative)	Typical council response for residential
Very Low	Less than 1 in 10,000 (< 1E-4)	Permitted, standard controls
Low	Approximately 1 in 10,000	Permitted, standard controls
Moderate	Approximately 1 in 1,000 (1E-3)	Permitted with engineered mitigation; conditions on drainage and earthworks
High	Approximately 1 in 100 (1E-2)	Rarely approved for new residential; extensive mitigation required or refusal
Very High	Greater than 1 in 100	Generally prohibited for new residential development

Most councils state that they accept new residential development only where the residual risk (after any proposed mitigation) is at Moderate or below. High and Very High classifications almost always result in refusal, redesign, or a referral to an independent peer reviewer before the council will make a decision.

The geotechnical engineer’s job is to get the risk classification as low as possible through design: slope re-grading, drainage works, retaining structures, and building envelope positioning. Some sites cannot be brought below High regardless of mitigation, and the honest advice from the engineer is to walk away or accept a non-residential land use.

Steep-slope construction implications

Once a site passes the geotech hurdle, the engineering demands of building on it drive cost:

Stepped footings. On a sloping site, footings step down the slope in increments rather than sitting on a flat plane. Each step requires additional concrete and formwork. On a 1:4 slope with a modest footprint, this adds two to four extra steps and material cost compared to a flat site.

Deeper foundations. If the upper soils are marginal or the bedrock is deep, footings need to penetrate to competent material. On steep clay sites this can mean going 600 mm to 1.2 m deeper than a standard footing.

Piled foundations. Where the soil is too unstable or the slope too steep for conventional footings, engineer-designed screw piles or bored concrete piers bear the load and eliminate the cut-and-fill problem. Piling adds significant cost but removes reliance on surface soil stability.

Retaining walls. Cut-and-fill earthworks require retaining walls on the cut face. Any retaining wall over 1 m in height in most states needs structural design and engineering certification. Walls over 1.5 m often require council approval independently of the DA.

Surface water management. Slope concentrates stormwater. Upslope neighbours’ runoff becomes your problem at the building platform. The geotechnical engineer and sometimes a drainage engineer will specify cut-off drains, subsoil drainage, and swale or pipe routes before any footings go in. Getting this wrong after construction is expensive and can trigger the very instability the report was designed to prevent.

Cut-and-fill versus piling tradeoff. Cut-and-fill is cheaper if the slope permits it and the excavated material can be disposed of economically. Piling avoids large volumes of fill and retaining, which is better on steep or geotechnically marginal ground but comes at higher upfront cost.

Cost implications

These figures are indicative for Class 1a residential. Site-specific conditions vary widely.

Item	Flat, stable lot	Moderate landslip site	High landslip site
Geotechnical report	$700-1,500 (AS 2870 classification)	$3,000-8,000 (full AS 1726 + AGS risk assessment)	$5,000-12,000+ (including peer review)
Standard slab/footings	$10,000-18,000	N/A (not applicable)	N/A
Stepped footings	N/A	$20,000-40,000	$35,000-60,000
Piled foundation	N/A	$30,000-60,000	$50,000-90,000
Retaining walls (1-2 m)	Nil or minor	$10,000-30,000	$20,000-60,000
Drainage engineering	$500-1,500	$5,000-15,000	$15,000-40,000
Rough total site premium	Baseline	+$40,000-80,000	+$80,000-150,000+

These are site-works premiums on top of the house construction cost. A builder quoting on a steep-slope DA site without pricing the slope premium is either padding the contingency silently or setting up a variation claim later.

State variance

Landslip and steep-slope controls are council-administered in most states. The overlay mapping and specific slope thresholds differ by council.

State	Active regions	How it is administered	Key trigger
NSW	Northern Beaches (7 geotechnical planning classes), Blue Mountains (slope constraint area under LEP s6.4), Wollongong (DCP Chapter E12), Ku-ring-gai, Hornsby	Council DCP or LEP; Section 10.7 certificate discloses constraint	Typically 15-18 degrees; overlay map
VIC	Dandenong Ranges (Yarra Ranges Council, Erosion Management Overlay EMO), Mornington Peninsula (Amendment C312morn added EMO7 for highly susceptible areas), Gippsland hills	Erosion Management Overlay (EMO) in council planning scheme; planning permit required for buildings and works inside EMO	EMO boundary (mapped, not always slope-degree based); scheme schedule sets requirements
QLD	Sunshine Coast hinterland (Landslide Hazard and Steep Land Overlay: Moderate, High, Very High hazard areas; Steep Land: 15-25%, >25% slope classes), Toowoomba escarpment, Gold Coast hinterland	Planning scheme overlay; code assessment or impact assessment depending on hazard level	Overlay mapping or steep land slope class
WA	Darling Ranges, Perth Hills (especially near Helena Valley, Kalamunda, Mundaring), Swan Coastal Plain clay-over-limestone zones	Local Planning Scheme; council policy; no single statewide overlay instrument	Council-specific; local policy often requires report for slopes >15 degrees in mapped areas
SA	Adelaide Hills corridor, Stirling, Aldgate, Crafers; mapped in the Planning and Design Code overlays	Planning and Design Code Hazard (Slope) Overlay; now statewide instrument	P&D Code overlay boundary
TAS	Hobart hillsides, Huon Valley, North West Coast; Landslide Planning Map (updated 2025 by Mineral Resources Tasmania)	Tasmanian Planning Scheme Landslip Hazard Code C15.0; hazard bands: Acceptable, Low, Medium, Medium-Active, High; geotechnical report per AS 1726-2017 and AGS 2007 required for Medium and above	C15.0 mapping bands

Council-specific DCPs are the most reliable source. Verify on the council’s mapping portal and check the instrument version date before relying on it.

What can go wrong

Assuming the tree cover means the slope is stable. Vegetation can mask old slip scarps. Tree roots can even hold a slope temporarily that will destabilise after clearing. Never let the green view from the street substitute for the geotechnical report.

Not budgeting the geotech report at feasibility. A full AS 1726 report with AGS risk assessment costs $3,000-8,000 and takes two to four weeks. If you or your client skips it at feasibility and the site turns out to be High risk, the redesign cost and delay will dwarf the report fee.

Ignoring upslope runoff. Surface water from upslope properties is a common trigger for shallow landslips. The drainage engineer and geotech need to model the full catchment, not just the lot. A cut batter left without drainage on a clay-rich hillside through the first wet season is a claim waiting to happen.

Treating the overlay as binary. “In the overlay” is not the same as “cannot build.” Most overlays permit residential development with engineered mitigation at Moderate risk classification. Early engagement with the council’s duty planner and a preliminary geotechnical assessment will tell you whether the project is viable before you spend design money.

Relying on an outdated report. Geotechnical reports have limited shelf lives. A report more than five years old on a site that has had earthworks, vegetation change, or drainage alterations nearby may not reflect current conditions. Councils may ask for a new report or a peer review of the old one.

The planning instrument hierarchy explains how landslip overlays sit inside the broader hierarchy: LEP or DCP in NSW, planning scheme in VIC and QLD, P&D Code in SA, LPS in TAS.

DCPs carry most of the landslip-specific requirements in NSW: the slope threshold, the required assessment level, and any conditions that apply at each risk classification.

Flood-prone area controls follow a parallel logic: overlay triggers a referral, a specialist report sets the risk classification, and the classification determines what conditions attach to the DA. If your steep-slope site also sits in a flood catchment, you will have concurrent referrals.

References

AS 1726-2017 Geotechnical site investigations, Standards Australia. Available via store.standards.org.au (verified 2026-05-23).
Australian Geomechanics Society, Practice Note Guidelines for Landslide Risk Management 2007 (AGS 2007c). Overview at landsliderisk.org (verified 2026-05-23).
Wollongong City Council DCP 2009 Chapter E12 Geotechnical Assessment of Slope Instability, wollongong.nsw.gov.au (verified 2026-05-23).
Northern Beaches Council Geotechnical Review, Geotechnical Planning Controls (Douglas Partners, 2023), northernbeaches.nsw.gov.au (verified 2026-05-23).
Mornington Peninsula Shire Amendment C312morn, Highly susceptible landslide areas, mornpen.vic.gov.au (verified 2026-05-23).
Sunshine Coast Planning Scheme 2014, Part 8 Overlays, Landslide Hazard and Steep Land Overlay, sunshinecoast.qld.gov.au (verified 2026-05-23).
Tasmanian Planning Scheme State Planning Provisions C15.0 Landslip Hazard Code, stateplanning.tas.gov.au (verified 2026-05-23).
Feasly, Geotechnical Reports in Australia: The Complete Developer’s Guide, feasly.com.au (verified 2026-05-23).