Soil classification and why your house slab quote depends on it

Two identical houses in two suburbs can have wildly different slab costs. The difference, usually, is the soil. Here’s why soil classification matters, what the AS 2870 site classes mean, and what your slab quote should actually depend on.

What AS 2870 is

AS 2870 is the Australian Standard for residential slabs and footings. It classifies sites from A (most stable) to P (most reactive) based on how much the soil moves with moisture changes, and prescribes minimum slab and footing requirements for each class.

The six classes that matter

• Class A — stable. Sand, rock. Minimum slab. Cheapest.
• Class S — slightly reactive. Slight movement expected.
• Class M — moderately reactive. The most common Sydney suburban classification.
• Class H1/H2 — highly reactive. Reactive clay sites. Heavier slab, deeper edge beams.
• Class E — extremely reactive. Significant movement expected. Substantial slab.
• Class P — problem sites. Engineered solution required (piles, deep footings).

Why it changes the price

A Class A slab on a sandy site might need 100mm slab, SL72 mesh, 300mm edge beam. A Class H2 slab on the same footprint needs 150mm slab, heavier reinforcement (often SL92), 600–800mm edge beam with extra bars. The concrete and reinforcement quantities double or triple.

Class P sites can need piles, or strip footings to rock — completely different engineering. The slab cost can be 3–5x a Class M equivalent.

Sydney soil patterns

Generally — northern and eastern Sydney sandstone areas tend to be Class S or M. Western Sydney clay belt is often Class H. Coastal sand areas can be Class A. But these are generalities — your specific site might be very different from your neighbour’s.

What you need to do

Get a soil report from a registered geotech engineer before you ask for slab quotes. The report classifies your site, identifies any unusual conditions, and gives the slab designer the information they need.

Without a soil report, any slab quote is a guess. We’ll happily provide an indicative range, but the real quote depends on the classification.

How to get one

Geotech soil reports for residential blocks typically cost $800–1,500 and take 1–2 weeks. We can refer you to Sydney geotech firms we’ve worked with. The investment pays for itself many times over — getting the slab wrong because you skipped the soil report leads to cracking, rectification work, and in extreme cases, structural compromise.