You've booked a soil test for your new home or extension, the report comes back, and there it is: a single letter — maybe M, H1, or E — that suddenly determines how your foundations are designed and, often, what they cost. So what does it actually mean?
Here's a plain-English guide from a structural engineering team to how site classification works under Australian Standard AS 2870, what each class means, and why getting it right protects your home from cracking and your budget from surprises.
A site classification describes how much the ground under your building is likely to move as soil moisture changes with the seasons. Reactive clay soils swell when wet and shrink when dry, and that constant "heave and shrink" is one of the leading causes of cracked slabs, sticking doors and wall cracks in Australian homes.
Classification follows AS 2870 – Residential Slabs and Footings, which is called up by the National Construction Code (NCC). It's based on a calculated value called the characteristic surface movement (ys) — essentially, the expected vertical ground movement in millimetres. The higher the ys, the more reactive the site, and the stronger (and usually more expensive) your footing system needs to be.
| Class | Reactivity | Typical ground movement (Ys) | What it usually means for footings |
|---|---|---|---|
| A | Stable — sand or rock | Little to none | Simple, standard footings and slabs |
| S | Slightly reactive clay | Up to ~20 mm | Slightly stiffened footings |
| M | Moderately reactive clay/silt | ~20–40 mm | Stiffened raft slab / deeper footings |
| H1 | Highly reactive clay | ~40–60 mm | Reinforced stiffened or waffle raft |
| H2 | Highly (very) reactive clay | ~60–75 mm | Heavily reinforced raft design |
| E | Extremely reactive | Over ~75 mm | Special engineered design (often piered) |
| P | "Problem" site | Varies | Site-specific engineered solution |
Class A is the most stable — sand and rock with little moisture-driven movement. As you move down the list, the ground gets more reactive and the engineering gets more involved. Class E sites are extremely reactive and demand carefully engineered footings.
Class P ("Problem") is a special case rather than a step up the reactivity scale. It's used for soft or unstable soils, sites prone to erosion, deep fill, slope-stability issues, abnormal moisture conditions, or any site that can't be sensibly classified by ys alone. A P-class site nearly always needs a bespoke engineered footing solution.
You might see a class written as M-D, H1-D, H2-D or E-D. The "D" stands for deep-seated moisture movement — typical of drier climates where the depth of soil moisture change (Hs) is 3 metres or more. It signals that ground movement extends deeper, which the footing design has to account for.
It's not a guess — it comes from testing and investigation, usually by a geotechnical engineer or soil tester:
Note that the classification can change if the site is cut, filled, or if trees, retaining walls or drainage are added within the zone that influences your footings — so it's a snapshot of the site as tested.
Your classification drives three things that matter to every build:
Australian geology changes street to street, which is why classification is site-specific:
The only way to know your site is to test it — a neighbour's classification is not a reliable guide to yours.
Trees are worth a special mention: their roots draw moisture from the soil, which can increase ground movement near footings (the standard accounts for this as tree-induced movement). Removing a large tree before building — or planting one too close afterward — can change how your soil behaves. If there are established trees on or near your block, flag them early; they can influence both classification and footing design.
QED Engineers designs compliant, cost-effective footing and slab systems to suit your site classification — from simple slabs on stable ground to engineered rafts and piered solutions for highly reactive sites — across [QLD, NSW, VIC & ACT]. Send us your soil report (or we can advise on arranging testing) for a fast fee proposal.
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This article is general information only. Your site classification and footing design must be determined for your specific site by qualified geotechnical and structural engineers in accordance with AS 2870 and the NCC.
What is the most common site classification? It varies entirely by location. In reactive-clay regions like South-East Queensland, H1 and H2 are common; in sandy or rocky areas, Class A or S is more typical. Your site must be tested to know.
Is a Class H1 or H2 site bad? No — it just means highly reactive clay that needs a properly engineered, reinforced footing system. Millions of homes sit safely on H-class sites; the key is designing for it rather than ignoring it.
Do I need a site classification for an extension? Usually yes. Any new footings need to be designed for the ground they sit on, so your engineer will need the site classification for the extension area.
How much does a site classification cost? It's typically a few hundred dollars for a standard residential soil test and report, carried out by a geotechnical engineer or soil tester. Complex or difficult-access sites cost more.
Can my site classification change? Yes. Cutting or filling the site, adding retaining walls or drainage, or removing/planting trees within the zone that influences your footings can change how the soil behaves and may require reassessment.
