Soil Classification Explained — A, S, M, H1, H2, E and P Sites
What your soil test result actually means, how each classification affects your slab design, and why a higher soil class can add thousands to your site costs.
Cost estimates only: Any building or upgrade figures in this guide are indicative, not fixed prices or quotes. Costs and inclusions vary by project, supplier, location and time. Confirm current written prices and specifications directly with your builder, suppliers and relevant trades.
Table of Contents
- The Australian Standard (AS 2870)
- What Each Classification Means
- How Soil Classification Affects Slab Design
- What a Soil Test Actually Involves
- Why Classifications Can Change
- Cost by Classification
- What to Check Before Signing
- Questions to Ask Your Builder
- Questions to Ask Before Buying the Land
- Common Mistakes
When you build a new home in Australia, one of the first site assessments your builder will order is a soil test. The result is a classification — A, S, M, H1, H2, E or P — that tells the engineer how reactive your soil is and how robust your slab needs to be. This single letter can add tens of thousands of dollars to your site costs, and it is one of the most common reasons buyers receive unexpected variation invoices after signing their building contract.
Understanding what each classification means, how it affects your slab design, and what it can cost — before you sign — is one of the most important steps you can take to avoid hidden site costs.
The Australian Standard (AS 2870)
All residential slab designs in Australia are governed by AS 2870, the Australian Standard for Residential Slabs and Footings. The standard classifies sites based on the reactivity of the soil — how much the soil swells when wet and shrinks when dry. This movement is what cracks slabs and walls if the slab is not designed to handle it.
The classification is determined by a geotechnical engineer who drills boreholes on your block, tests the soil samples in a laboratory, and issues a site classification report. The report is then used by the structural engineer to design the appropriate slab.
What Each Classification Means
| Classification | Soil Type | Reactivity | Typical Slab Required |
|---|---|---|---|
| A | Stable (sand, rock, gravel) | Very low | Standard waffle pod or raft slab |
| S | Slightly reactive clays | Low | Standard waffle pod or raft slab |
| M | Moderately reactive clays | Moderate | Reinforced waffle pod or raft slab |
| H1 | Highly reactive clays | High | Heavily reinforced slab, possible beam stiffening |
| H2 | Highly reactive clays (severe) | Very high | Heavily reinforced slab with deep beams |
| E | Extremely reactive clays | Extreme | Engineered raft or pier and beam slab |
| P | Problem site (soft, filled, contaminated) | Variable / unstable | Fully engineered solution — piers, piles, or raft |
Note
How Soil Classification Affects Slab Design
The more reactive the soil, the more movement the slab needs to withstand. This translates directly into slab complexity and cost:
- A and S class — a standard waffle pod slab is usually sufficient. This is often included in the builder's base price.
- M class — the slab needs additional reinforcement. Some builders include this in the base price; others charge $5,000–$15,000.
- H1 and H2 class — the slab needs deeper beams, more steel reinforcement, and sometimes a raft slab instead of a waffle pod. This typically adds $15,000–$40,000+.
- E class — the slab needs a fully engineered raft slab or pier and beam system. This can add $30,000–$60,000+.
- P class — the site requires a fully engineered solution, which may include driven piles, bored piers, or a thick raft slab. Costs can exceed $50,000–$80,000+.
What a Soil Test Actually Involves
A geotechnical engineer visits your block and drills a series of boreholes — typically 2 to 3 for a standard residential block. Soil samples are taken at various depths and sent to a laboratory for analysis. The engineer assesses:
- Soil type and plasticity (how much the clay content causes swelling and shrinking)
- Depth of the reactive layer
- Moisture conditions
- Presence of fill, organic material or contamination
- Groundwater levels
The result is a site classification report that specifies the classification (A through P) and provides recommendations for the slab design. The report is usually valid for the specific borehole locations — conditions elsewhere on the block may differ.
Why Classifications Can Change
One of the most common sources of unexpected site costs is a change in soil classification between the initial test and the start of construction. This can happen because:
- The initial test is based on limited boreholes — conditions between boreholes may differ.
- The builder may order a more detailed test after site scrape, revealing deeper reactive layers.
- Fill placed by the developer may not have been properly identified in the initial test.
- Groundwater conditions may change between the test date and construction.
- The builder's excavation may reveal rock, soft spots or contamination not detected by the boreholes.
Provisional Sums
Many builders quote the slab as a 'provisional sum' — an estimate based on an assumed classification. If the actual classification turns out to be higher, you pay the difference. Always check whether your slab is a fixed price or a provisional sum.
Cost by Classification
What to Check Before Signing
- Order a soil test before signing the building contract if one has not been done.
- Check the classification on the soil test report — A, S, M, H1, H2, E or P.
- Ask your builder whether the slab price is fixed or a provisional sum.
- If it is a provisional sum, ask what classification it assumes and what the upgrade cost is for each higher class.
- Ask whether the builder has built on similar soil in the same estate.
- Check if the developer has any geotechnical reports available for the estate.
- Budget for a potential upgrade if the classification is M or higher.
Questions to Ask Your Builder
- What soil classification is the slab price based on?
- Is the slab price fixed or a provisional sum?
- If the classification changes during construction, who pays the difference?
- What is the upgrade cost for each higher classification?
- Have you done a soil test on this specific block, or are you estimating?
Questions to Ask Before Buying the Land
- Is there a geotechnical report available for this block or the estate?
- What soil classification is typical for this area?
- Are there known reactive soil areas in this suburb?
- Does the developer have any fill or contamination reports?
- Will a soil test be done before or after I sign the building contract?
Common Mistakes
- Signing a building contract before a soil test is done, then receiving a variation for a higher classification.
- Not understanding that the slab is quoted as a provisional sum, not a fixed price.
- Assuming the initial soil test is definitive — conditions can change once excavation begins.
- Not budgeting for a slab upgrade on M, H or E class sites.
- Not checking whether the developer placed fill on the block that could affect the classification.
Frequently Asked Questions
Practical Takeaway
Your soil classification is one of the single biggest determinants of your site costs. An A or S class site may cost nothing extra, while an E or P class site can add $50,000 or more. The most important step is to order a soil test before signing your building contract, and to check whether the slab price is fixed or a provisional sum. If it is a provisional sum, budget for the possibility of an upgrade — especially in areas known for reactive clay soils.
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