Site Classification Explained: A, S, M, H1, H2, E and P Ratings
Every residential site in Australia gets a classification letter that quietly determines almost every foundation decision that follows. Here’s what each rating actually means.
What Site Classification Means
Under AS 2870 (the Australian Standard for residential slabs and footings), every site is assigned a classification based on how much its soil is expected to move due to seasonal moisture change, along with any other complicating site factors. This classification, determined from a geotechnical soil report, is the single biggest input into what foundation system your engineer will specify.
Class A and S: Stable Sites
Class A sites have stable, non-reactive soil — typically sand or rock — with minimal expected ground movement. Class S sites are slightly reactive, with limited but measurable ground movement expected. Both classifications generally allow for more straightforward, cost-effective foundation systems, since the ground itself isn’t expected to move enough to demand significant extra design tolerance.
Class M: Moderately Reactive
Class M sites have moderately reactive soil, with meaningful but manageable seasonal ground movement expected. This classification sits in the middle of the scale and often still allows a relatively standard foundation approach, though with more design consideration than Class A or S sites require.
Class H1, H2 and E: Reactive to Extreme
Class H1 (highly reactive) and H2 (very highly reactive) sites have soil that expands and contracts significantly with seasonal moisture change — common across much of Melbourne’s basaltic clay growth corridors. Class E (extremely reactive) represents the most severe end of this scale. These classifications typically require a foundation system specifically engineered to tolerate substantial ground movement, most commonly a waffle slab.
Class P: Problem Sites
Class P doesn’t describe soil reactivity directly — it identifies sites with other complicating factors: significant fill, poor drainage, mine subsidence risk, landslip potential, soft or collapsing soils, or proximity to large trees affecting soil moisture. Class P sites need specific engineering attention addressing whatever the identified issue actually is, which varies considerably from site to site.
How Classification Drives Foundation Design
Once your site’s classification is known, your structural engineer uses it — alongside your building’s specific loads and layout — to design an appropriate, certified foundation system. See our waffle vs raft slab comparison for how this plays out in practice for the two most common residential slab systems, and our guide to how soil type affects your foundation for the bigger picture.
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Frequently Asked Questions
How do I find out my site's classification?
A geotechnical soil report, carried out specifically on your site, determines the classification under AS 2870.
Is Class A the best classification to have?
It generally means the most stable, straightforward ground conditions for foundation design, though every classification can be built on successfully with the right engineering.
What's the difference between H1, H2 and E classifications?
They represent increasing levels of soil reactivity — how much the ground is expected to expand and contract seasonally — with E being the most extreme.
Can a site have more than one classification?
A site gets one overall classification, though a Class P designation (problem site) can sometimes be assigned alongside underlying reactivity considerations.
Does site classification affect cost?
Yes, generally — more reactive classifications (H1, H2, E) typically require more robust, higher-cost foundation systems than stable Class A or S sites.
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