What is AS 2870 site classification?
Before a certifier can sign off your footing design, your block needs a site classification to AS 2870. Here's what that means in plain English.
What a site classification actually is
A site classification rates how much your ground is likely to move as its moisture content changes through the seasons. Clay soils swell when wet and shrink when dry; that movement is what cracks slabs and footings if they aren't designed for it. AS 2870 — the Australian Standard for residential slabs and footings — sorts every site into a class based on that expected movement.
The classification is the starting point your engineer uses to design a footing system that won't crack, and the document your certifier needs before construction can be approved.
The soil classes
There are seven classifications under AS 2870. The further down the list, the more the ground moves and the more engineering your footings need.
| 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. |
Much of Sydney falls into Class M or H — reactive clays are common across the basin. For a deeper look at each class, see our soil classes explained guide.
When do you need one?
You'll generally need a site classification for a new dwelling, a knock- down rebuild, a granny flat or secondary dwelling, and most additions that add new footings. Your builder, engineer or certifier will ask for it before footing design is finalised.
How it's done
We attend the site and put down boreholes (usually by hand auger) or a dynamic cone penetrometer test to log the soil profile and assess its reactivity. Combined with the site's geology and any fill or drainage issues, that gives us the classification — delivered as a concise, council- and certifier-ready report, typically within a few days.
What it costs
For most residential blocks in Sydney a site classification sits between roughly $600 and $1,200, depending on access, how many boreholes are needed and how quickly you need it. It's a small cost next to the expense of footings that fail — or a DA held up waiting on the report.
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.