What is an AS 2870 site classification — and why does every new home need one?
If you're building or developing residential land in Queensland or New South Wales, you'll need a site classification before a certifier can approve your footing design. But beyond the regulatory requirement, understanding what a site classification actually tells you — and when to get one — has real implications for project cost and risk.
The problem site classifications solve
Much of the residential land across South East Queensland and coastal New South Wales sits on reactive clay soils. These clays swell when wet and shrink when dry. When the ground beneath a house moves unevenly, it puts stress on the footing and the structure above it — leading to cracked masonry, racking door frames, and in worse cases, structural damage to the slab itself.
Reactive soil damage is one of the most common causes of building defects in Australia. A site classification quantifies that risk and determines the footing system needed to manage it.
What AS 2870 is
AS 2870 — Residential Slabs and Footings — is the Australian Standard governing footing design on reactive soils. In use since 1986, with the current edition published in 2011, it provides a method for estimating how much the ground at a site is likely to move under expected moisture changes.
That estimated movement is called the characteristic surface movement, or ys. It's calculated from the shrink-swell index (Iss) — a laboratory measurement of how much a clay sample swells and shrinks under controlled conditions. The ys value determines the site class, and the site class determines the footing.
The site classification system
| Class | Surface movement | Typical soil |
|---|---|---|
| A | 0 – 20 mm | Non-reactive — sands, gravels, rock |
| S | 0 – 20 mm | Slightly reactive clay |
| M | 20 – 40 mm | Moderately reactive clay |
| H1 | 40 – 60 mm | Highly reactive clay |
| H2 | 60 – 75 mm | Very highly reactive clay |
| E | > 75 mm | Extremely reactive |
| P | — | Problem sites: filled land, soft soils, unstable ground, or conditions outside the standard's scope |
Moving up the scale means a heavier, more expensive footing system. A Class P site can't be addressed by AS 2870 alone — it requires specific geotechnical engineering input before any footing design can proceed.
How a classification is determined
Two approaches are used in practice:
Laboratory testing — Soil samples are collected on site and tested for shrink-swell index. Results feed directly into the ys calculation. Required for complex, disputed, or high-consequence sites.
Desktop assessment — A geotechnical engineer reviews existing data — nearby borehole records, soil survey data, geological mapping — and derives a probable classification without drilling on the specific lot. Well-suited to standard residential lots, particularly early in the development process before site access is established.
Getting it right matters
An under-classified site — where actual soil reactivity is higher than the classification suggests — puts the dwelling at risk of footing movement and damage. An over-classified site adds unnecessary cost to every footing built on that land.
Damage from an under-classified site can cost tens of thousands of dollars to rectify. The disputes that follow over who is responsible are common, drawn out, and rarely straightforward. Getting the classification right — based on good data, applied carefully — avoids both problems.
When to get one
A site classification is most valuable before land is purchased, before a design is committed to, and before physical investigation is scoped. Ground conditions are one of the hardest development risks to price once a site has been acquired. The earlier that picture exists, the more useful it is.
For developers assessing multiple lots, or making acquisition decisions across a portfolio, early desktop assessments provide the ground intelligence needed to price risk, prioritise investigation, and avoid sites that are likely to be problematic before money is committed.
What LayeredGeo does
LayeredGeo automates the data integration behind desktop site classification to produce an indicative AS 2870 site classification based on reactive soil mapping, geological data, and historical investigation records.
Get in touch to find out how we can support your next project.
LayeredGeo is an automated geotechnical desktop reporting platform serving the residential development sector in Queensland and New South Wales.