The short answer: a soil test, also called a site classification, is when a geotechnical engineer takes samples from your block, tests them and gives the site a class under the Australian Standard AS 2870. The class tells your engineer how much the ground is likely to move as it wets and dries, and the slab and footings are designed to suit (Intrax, Site classification FAQs). It is one of the first steps for a new home, and it matters for many extensions too.
We build new homes and extensions across Gawler, Angle Vale, Two Wells, Virginia and Adelaide's north. This guide explains what the soil report says and why it affects the build.
Why the soil matters
Clay soils swell when they get wet and shrink when they dry out. A slab sitting on that ground moves with it, and if the slab and footings are not designed for the amount of movement, you can end up with cracked walls, sticking doors and cracked tiles. Much of Adelaide sits on reactive clay, and Adelaide University research published in September 2026 looked at how drier, hotter conditions affect home foundations (Adelaide University news).
What the site classes mean
AS 2870 puts each site into a class based on its expected surface movement (Ys), measured in millimetres (Intrax):
| Class | Ground | Expected surface movement |
|---|---|---|
| A | Mostly sand and rock | Little or none |
| S | Slightly reactive clay | 0 to 20 mm |
| M | Moderately reactive clay or silt | 20 to 40 mm |
| H1 | Highly reactive clay | 40 to 60 mm |
| H2 | Very highly reactive clay | 60 to 75 mm |
| E | Extremely reactive clay | More than 75 mm |
| P | Problem site | Not a movement band |
The further down the table, the more the slab and footings have to do: higher classes need deeper, stiffer footings, which means more concrete and steel. Class P covers problem sites, such as soft soils, loose sands and abnormal moisture conditions, where an engineer has to design for the specific ground.
What can push a block into class P
- Fill. Newer estates are often cut and filled to level the blocks. Fill that was not placed and compacted under controlled conditions can make a site a problem site.
- Trees. Large trees close to the house draw moisture out of clay and make it move more than it otherwise would.
- Abnormal moisture. Leaking pipes, poor drainage, old dams, or water pooling against the house.
- Soft or loose ground. Ground with low bearing capacity, or soil that is prone to erosion.
If your block is in a newer estate around Gawler or the northern growth areas, ask the developer for any fill or compaction certificates. Your engineer will want them.
How a soil test works
- A geotechnical engineer visits the block and takes soil samples, usually from a few spots across where the house will sit.
- The samples are tested to see what the soil is made of and how reactive it is.
- You get a report with the site class and notes on anything unusual, such as fill or trees.
- Your structural engineer uses the report to design the slab and footings that go into the building approval documents.
Do you need one for an extension?
Often, yes. An extension needs footings designed for the ground it sits on, and the new section has to work alongside the existing house so the two do not move apart. A soil report may already exist from when the house was built. If it does, your engineer can tell you whether it is still usable. Our ground-floor vs second-storey extension guide covers how the footings affect that decision.
Protecting the slab once you have moved in
- Keep stormwater away from the house: downpipes connected, paving falling away from the walls.
- Fix leaking taps and pipes near the house quickly.
- Avoid flooding garden beds right against the slab, and plant large trees well away from the house.
- Keep watering steady through summer rather than soaking and drying out the ground near the walls.
How we handle it at ASP Build & Co
On our custom home builds and home extensions, the design, engineering and council approvals are managed as part of the job, so you are not chasing reports yourself. Every project starts with a free consultation and an itemised quote. If you have a block around Gawler or the northern suburbs and want to know what it will take to build on it, get in touch.
Common questions
What is a soil test for building?
It is a site investigation by a geotechnical engineer, who takes soil samples, tests them and gives the block a site class under AS 2870. The class is used to design the slab and footings.
What does site class M or H1 mean?
Class M is moderately reactive clay or silt, with expected surface movement of 20 to 40 mm. Class H1 is highly reactive clay, with 40 to 60 mm. The higher the class, the stronger the slab and footings need to be.
What is a class P site?
A problem site, where the ground needs special engineering. It can be caused by uncontrolled fill, soft soils, loose sands, abnormal moisture conditions or the effect of trees.
Does every new home need a soil test?
A site classification is one of the first steps for a new home, because the slab and footings are designed to suit the site class. For an extension, ask your engineer whether an existing report can be used.
Related pages
Sources
- Intrax Consulting Group, Site classification FAQs (AS 2870 site classes)
- Adelaide University news (September 2026): stronger home foundations needed to withstand climate change
Rules and published figures checked on 19 October 2026. Third-party cost ranges are not ASP Build & Co prices; we quote every job individually after a site visit.
