A geotechnical investigation tells you what's actually under the ground before someone designs a footing to sit on it. On a flat suburban block in stable soils, that may not amount to more than a routine soil test. On a sloping block, a coastal site, or anywhere that's been disturbed by past works, it's some of the most consequential money you'll spend in due diligence — because the gap between a Class M and a Class P site can shift a slab budget by tens of thousands of dollars.

A residential geotech report does two main jobs. First, it produces a site classification under AS 2870-2011 "Residential slabs and footings" — a single letter or short code that drives footing design across the build. Sites range from Class A (sand or rock with negligible movement) through Class S (slightly reactive), M (moderate), H1, H2 (highly and very highly reactive clays), E (extreme) to Class P — the "problem" site that needs engineered design because of slope, fill, soft soils, abnormal moisture, mine subsidence, or anything else outside the standard. A "-D" suffix indicates deep-seated moisture movement, typical in dry climates where the design depth of suction change reaches three metres or more. Volume builders tend to assume Class M or H1 in their headline pricing; H2 and E sites typically add $15,000–$55,000 to footing costs, and Class P sites can run to $200,000 or more when screw piles, bored piers or waffle voids with deep edge beams are required.

Second, on sloping or hazardous sites, a geotech report produces a landslide risk assessment under the Australian Geomechanics Society's Landslide Risk Management 2007 guidelines. AGS 2007 grades risk in two ways — Loss of Life (an annual probability that a person at the site is killed by a landslide event), and qualitative property loss zones from low through to very high or extreme. Multiple Australian councils — Wollongong, Lismore, Pittwater, Mornington Peninsula, Hobart, Adelaide Hills among others — have geotechnical risk management policies, and some will refuse or heavily condition development on lots in the higher-risk zones.

Cost depends on scope. A Category 1 report is essentially a desktop check with minimal fieldwork. Category 2 is the typical residential investigation — a few boreholes, lab tests on samples, and footing recommendations. Category 3 is the most detailed, with multiple boreholes, groundwater monitoring, and slope-stability modelling. Indicative 2025–26 prices: a basic soil test plus AS 2870 classification around $900–$2,500; a Category 2 investigation on a moderately sloping block $3,500–$8,000; a full Category 3 AGS landslide risk assessment $8,000–$25,000 or more.

A geotech report typically takes one to three weeks once a drilling crew is booked, and on sloping sites you should allow 6–12 weeks for groundwater monitoring and stability modelling. Don't sign an unconditional contract for a sloping or fill-affected block without one in hand, or without a contract condition that lets you obtain one.

Questions worth asking the seller:

  • Is there an existing geotechnical report on this lot — or a subdivision-stage report covering the estate?
  • Has the lot been filled or benched? If so, was the fill compacted in layers and certified, or is it uncontrolled fill?
  • Are there any council-recorded landslip events, past landslides on neighbouring lots, or unusual drainage issues?
  • Does the relevant council have a Geotechnical Risk Management Policy or a Landslide Hazard map covering this lot?
  • Has the lot ever been used as a quarry, or filled to level a depression?
  • Are there mature trees that have been recently removed (residual moisture deficit in reactive clays can drive years of heave after removal)?

Who can help. A geotechnical engineer — Chartered Professional Engineer (CPEng) on the National Engineering Register, in Queensland additionally Registered Professional Engineer of Queensland (RPEQ), and ideally a member of the Australian Geomechanics Society. For landslide work, look for someone who has authored AGS-compliant reports before and (in landslide-prone council areas) someone on the council's approved consultant register. Your structural engineer and architect will both rely on the geotech findings to set footing and design parameters, so engage the geotech early — before the design moves into detail.

The most expensive geotechnical surprise is the one discovered during construction. Spending $2,500 up front to avoid a $50,000 footing variation mid-build is one of the better trades available to a vacant-land buyer.

This article is general information only — a starting point for your own questions, not engineering, geological or professional advice. Soil conditions, site classifications and landslide risk vary by lot and are subject to change with vegetation, drainage and works on neighbouring land. Always engage a Chartered Professional Engineer with geotechnical experience appropriate to the site, and request from the seller any prior geotechnical or subdivision-stage reports. Independent advice should be obtained before making any property decision.