What Do Site Classes A, S, M, H1, H2 and E Mean?
Owners and buyers reading soil reports
A generally describes sand and rock sites with little or no moisture-related ground movement; S is slightly reactive, M moderately reactive, H1 and H2 highly reactive, and E extremely reactive. H2 has the higher expected movement description of the two H classes. These classes concern ground behaviour under normal-site assumptions, not a house-condition grade or measured floor distortion.
A site class describes ground behaviour relevant to footing design. It is not a house-condition grade. Classes A, S, M, H1, H2 and E distinguish sites by their expected response to moisture under normal-site assumptions. The classification must be read with the soil profile, the depth of movement and the investigation findings.
The letters in plain English
Class A describes most sand and rock sites with little or no ground movement from moisture changes. S is slightly reactive; M is moderately reactive; H1 and H2 are highly reactive, with H2 representing the higher expected movement description; E is extremely reactive. These are descriptions of potential ground response, not proof that a house on one class has moved by a particular amount.
Class A does not mean nothing can go wrong. Loose sand, erosion, fill or inadequate support raise different questions. Adequate bearing capacity is still needed in the sand context, and other site problems may require Class P rather than a normal reactivity classification.
What does the number in millimetres represent?
Characteristic surface movement is the movement of the reactive site's surface between characteristic dry and wet conditions without the building and without load effects. This design quantity distinguishes reactive site classes.
It is not the highest-to-lowest reading from a house floor survey. A building changes loading, moisture conditions and the interaction with the ground. Comparing a floor's present unevenness directly with a site-class band confuses two different quantities.
Why some reports have a D suffix
The classification also recognises deep-seated moisture change. A suffix such as M-D distinguishes the depth aspect of the classification, not just a larger version of an ordinary floor defect. Climate and the soil profile influence that assessment; the suffix is not a drilling-depth instruction for the owner.
Questions worth asking about your report
Ask which investigation and classification method were used, which soil layers were found, and whether known cutting, filling, trees or abnormal moisture were included. Confirm that the report relates to the actual building footprint and proposed work. If the design is being reused after substantial site changes, ask whether its assumptions remain valid.
Leave classification and interpretation to a suitably qualified practitioner. Your useful contribution is the site history and records, including what remains unknown, rather than choosing a class from the appearance of surface soil.