What the site class actually measures
Clay soils swell when they get wet and shrink when they dry out. A footing sitting on clay moves with it, and a house on a footing that moves too much cracks. The site class is an estimate of how much the ground surface is likely to move between its wettest and driest states over the life of the house, which AS 2870 calls the characteristic surface movement.
Sand and rock barely move, so they get the easiest class. Reactive clays, the kind that crack in a dry summer and turn to putty in a wet winter, move a lot, so they get the harder classes and a stiffer footing to match.
The classes, from easiest to hardest
AS 2870 uses seven classes. The first six are ranked by expected movement; the seventh is for sites that need their own design.
- Class A: most sand and rock sites, with little or no ground movement from moisture changes.
- Class S: slightly reactive clay sites, with only slight ground movement (up to about 20 mm).
- Class M: moderately reactive clay or silt sites, with moderate movement (about 20 to 40 mm).
- Class H1: highly reactive clay sites (about 40 to 60 mm).
- Class H2: highly reactive clay sites with more movement again (about 60 to 75 mm).
- Class E: extremely reactive sites, with more than about 75 mm of expected movement.
- Class P: problem sites, where the class cannot be picked off the table. Fill, soft soil, a slope, trees close to the footings or unusual moisture conditions are the usual reasons.
What the letter means for your footing
The site class is the input to the footing design in AS 2870. A Class A or S site can usually take a standard slab or strip footing straight out of the standard's tables. An M site needs a stiffer slab, H1 and H2 stiffer again, and an E site often needs an engineered footing that is designed rather than looked up. None of these is a fail: a house can be built on any of them. The class simply tells the designer how much stiffness to build in.
A suffix of D, as in M-D or H1-D, means the movement is expected to be deep-seated rather than near the surface, which changes some of the design assumptions. Your designer will know what to do with it.
How a site is classified
The classification starts with boreholes on the block, usually two to about 2.5 metres for a house, which is what AS 2870 expects for a residential footing. A field engineer logs the soil as it comes out of the ground: what it is, how stiff or dense it is, how wet, and whether there is fill, rock or groundwater. Samples go to the laboratory where the class depends on how reactive the clay is, and the engineer combines the profile, the test results and the local geology into a class with the reasoning written down.
The boreholes are discrete points. A good report says what was found in each hole and what that means for the footing between them, and tells you when to call if the ground looks different once excavation starts.
Who asks for one, and when
Nearly every new house, extension, granny flat and garage on a slab or strip footings needs a site classification before the footings can be designed, so it is usually the builder, the slab designer, the structural engineer or the certifier who asks. The earlier it is done the better: a classification before the design is finished means the footing is designed once, for the real ground, rather than redesigned after a surprise.
For a standard residential site the fee is fixed, $890 + GST in Sydney Metro and fixed by zone across regional NSW, and it can be accepted online from the postcode.
Questions
Last checked 9 September 2026. Written by the engineers at Australian Ground Sciences, who prepare and sign the reports.