Soil classes explained: A to P
From stable Class A sand to extremely reactive Class E clay, here's what each AS 2870 soil class means for your site and your footings.
How the classes work
Under AS 2870, every residential site is given a class based on how much the ground surface is expected to move as moisture changes. Stable sites barely move; highly reactive clays can heave and shrink dramatically. The more movement, the more your footings have to do to keep the building stable.
| Class | Reactivity | Surface movement | What it means for footings |
|---|---|---|---|
| A | Acceptable / stable | 0–20 mm | Simple, economical footings — the easiest sites to build on. |
| S | Slightly reactive | 0–20 mm | Standard slab or strip footings, lightly engineered. |
| M | Moderately reactive | 20–40 mm | Stiffened raft slabs or deeper edge beams to resist movement. |
| H1 | Highly reactive | 40–60 mm | More heavily engineered slabs, often with deeper beams or piers. |
| H2 | Highly reactive | 60–75 mm | Substantial slab design or piered systems; higher build cost. |
| E | Extremely reactive | > 75 mm | Specialised, heavily reinforced footing systems required. |
| P | Problem site | Variable | Site-specific engineering design; may need ground improvement. |
What this means for your build
The class directly drives footing cost. A Class A or S site is the cheapest to build on; Class H1, H2 and E sites need progressively more engineering — deeper beams, more steel, sometimes piers — which adds to the slab cost. A Class P site needs a fully bespoke design.
Most of Sydney's clay-based suburbs land in Class M or H. Knowing your class early lets you budget the footings accurately instead of being surprised at construction stage.
How your class is determined
The class comes from a site investigation under AS 2870 — boreholes or a cone penetrometer test to log the soil, combined with local geology and any fill, drainage or slope issues. Read our site classification guide for the full process.
Talk to a soil & site expert
We prepare council-ready soil, geotechnical and DA reports right across Sydney. Call with your site address for clear advice.