July 15, 2026

Site Classification Explained: Class A to Class E

Quick answer. Site classification is a rating under the Australian Standard AS 2870 that describes how much the soil on a site is expected to move with changes in moisture, from Class A (stable) to Class E (extremely reactive). It is determined by a geotechnical soil test and it decides the footing system a home needs. Most Melbourne sites are reactive clay, commonly Class M, H1, or H2, which is why footing design matters so much across the city.

Almost every set of footing drawings for a Melbourne home starts with one letter, the site class. It is the single piece of information that most influences how the footings are designed, yet it is often the least understood. This guide explains what each class means, how the classification is worked out, and why it drives the engineering for a new build or an extension.

What Is Site Classification?

Site classification is the rating that describes how reactive the soil on a building site is, meaning how much it swells when wet and shrinks when dry. Reactive clay soils move as their moisture content changes through the seasons, and that movement pushes and pulls on the footings above. AS 2870, the Australian Standard for residential slabs and footings, sets out the classes so that a structural engineer can design footings that tolerate the expected movement without the building cracking.

What Are the Site Classes From A to E?

The classes run from A, the most stable, to E, the most reactive, with P reserved for problem sites that do not fit the normal scale. The table below sets out each class and the surface movement it represents.

Class	Soil behaviour	Characteristic surface movement
A	Stable, non-reactive. Sand or rock.	Little or no ground movement
S	Slightly reactive clay.	Up to about 20 mm
M	Moderately reactive clay or silt.	About 20 mm to 40 mm
H1	Highly reactive clay.	About 40 mm to 60 mm
H2	Highly reactive clay.	About 60 mm to 75 mm
E	Extremely reactive clay.	More than about 75 mm
P	Problem site (soft soil, fill, mine subsidence, or unusual conditions).	Assessed individually

Each step up the scale means the footings have to work harder to keep the building stable. A footing system that performs perfectly on a Class M site can crack and move on a Class H2 site, which is why the class is confirmed before the footings are designed, not after.

How Is a Site Classification Determined?

A site classification is determined by a geotechnical soil test, in which boreholes are drilled and the soil profile is examined and tested for reactivity and depth of the moisture-affected zone. A geotechnical engineer assigns the class in a soil report, which the structural engineer then uses as the basis for the footing design. The class cannot be reliably guessed from the surface, because two neighbouring sites can differ, so a soil report is the starting point for any build with a concrete slab.

Why Does Site Classification Matter for Footings?

Site classification matters because it dictates the type, size, and reinforcement of the footing system. A more reactive class needs a stiffer, deeper, or more heavily reinforced footing to resist the ground movement, and sometimes a different system entirely. A structural engineer designs the footing to AS 2870 for the specific class, so that seasonal soil movement is accommodated and the building above stays serviceable.

Class A and S sites often suit a standard slab with modest edge beams.
Class M, H1, and H2 sites need progressively stiffer and deeper footing systems.
Class E and P sites can need specific engineering solutions such as piered or suspended footings.
The wrong footing for the class is a common cause of cracking and movement in Melbourne homes.

What Is the Most Common Site Classification in Melbourne?

The most common site classifications in Melbourne are the reactive clay classes M, H1, and H2, because much of the city sits on clay soils. The outer south-eastern and north-western suburbs in particular are known for highly reactive H2 ground. This is the reason footing design, drainage, and the management of moisture around a home carry so much weight in Melbourne compared with regions on sandy or rocky soils.

Frequently asked questions

What does my site classification mean?

Your site classification is a rating under AS 2870 that describes how much the soil on your site moves with changes in moisture, from Class A (stable) through to Class E (extremely reactive), with P for problem sites. It is the main factor that determines the footing system your home needs, and it is assigned by a geotechnical soil test.

Who determines the site classification?

A geotechnical engineer determines the site classification from a soil test, drilling boreholes and assessing the soil profile and its reactivity, then recording the class in a soil report. A structural engineer uses that class to design the footings to AS 2870. The two roles work together, the geotechnical report first, then the structural footing design.

What is the most reactive site classification?

Class E is the most reactive class on the standard scale, representing extremely reactive clay with characteristic surface movement greater than about 75 mm. Sites that do not fit the normal scale, such as soft soils, uncontrolled fill, or mine subsidence areas, are classified P for problem site and assessed individually by an engineer.

Does a higher site classification change the footing design?

Yes. A higher site classification means the footings must resist greater ground movement, so the engineer designs a stiffer, deeper, or more heavily reinforced footing system, and on the most reactive sites a different system such as piered footings. Getting the footing right for the class is what keeps the building stable through the seasons.

Can a site classification change over time?

The underlying soil class does not change, but the way a site behaves can worsen if moisture is not managed, for example through poor drainage, plumbing leaks, or large trees drawing water from under the footings. This is why an engineer considers site drainage and moisture management alongside the classification when designing footings.

General guidance only. This article is general education, not certified structural engineering advice. Every site is different and the classification must come from a soil test for the specific property. For footing design based on your site classification, engage a chartered structural engineer. Principal Built Engineering can design your footings to AS 2870.

Related PBE services. See residential structural engineering for new builds and extensions, and when to worry about cracks in walls if footing movement is already showing.

Building on a reactive site in Melbourne?

Principal Built Engineering designs footings to AS 2870 for the site classification of your property across Melbourne and Victoria, with documentation delivered in approximately two weeks.

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Last reviewed: July 2026 by Principal Built Engineering, structural engineers, Melbourne.

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