Guide

Soil classification and what it does to your slab

The most expensive part of a house that nobody looks at is underneath it. Site classification is why two identical houses on the same street can have different slab prices.

Last updated 13 September 2026 · Written by the Better Builds team

Before a footing system can be designed, the site has to be classified. In Australia this is done under AS 2870 Residential Slabs and Footings — the National Construction Code requires the foundation to be classified in accordance with it — on the basis of a geotechnical investigation — usually boreholes and laboratory testing of the soil recovered.

What the classification measures

Principally how much the ground moves as its moisture content changes. Reactive clays swell when wet and shrink when dry, and that movement is what cracks a slab and the walls above it. A footing system is designed to tolerate the expected movement rather than to prevent it.

The classes

Class A — most sand and rock sites, little or no ground movement. Class S — slightly reactive clay. Class M — moderately reactive. Class H1 and H2 — highly reactive. Class E — extremely reactive. Class P — problem sites, which includes fill, soft soils, landslip, mine subsidence and sites affected by trees or abnormal moisture conditions.

The step from a low-reactivity class to a high one means more concrete, more steel and deeper beams. Class P is a different conversation again, because it is not a reactivity rating so much as a statement that the site needs specific engineering.

Why neighbouring blocks differ

Cut and fill from the original subdivision, a removed tree, a filled watercourse, or simply a change in the underlying geology across a street. This is also why a soil test on the block next door is not a substitute for one on yours.

How this shows up in a quote

Usually as a provisional sum for site costs, because the classification is often not available when the quote is written. That is reasonable — but it means the figure is an estimate, and a thin estimate makes a quote look cheaper without changing the ground. See provisional sums and prime cost items and what site costs are.

What to do about it

Get the soil test done early, ideally before you sign, and have the site costs priced against the actual classification rather than an allowance. It converts the largest variable in a residential build into a known number, and it is inexpensive relative to what it de-risks.