Quick answer: AS 2870 site classes describe expected residential foundation behaviour, particularly movement associated with moisture change. A, S, M, H1, H2 and E progress from little or no reactivity to extreme reactivity. Class P identifies a site requiring special consideration. The class is not a suburb rating and must be determined from a project-specific site assessment.
Why site classification matters
Residential footings and slabs interact with the ground beneath them. Reactive clay can shrink as it dries and swell as it becomes wetter, producing movement across the building footprint. The structural engineer or footing designer uses the site classification, expected movement and other investigation information to design a footing system appropriate to the site.
The National Construction Code requires foundations for relevant residential footings and slabs to be classified in accordance with AS 2870. The classification report does not replace structural design; it provides key ground information for that design.
A, S, M, H1, H2, E and P site classes
Little or no moisture-related movement
Commonly associated with stable sand or rock conditions. Other factors such as fill, slope or bearing conditions still require consideration.
Slightly reactive
Slight ground movement from seasonal moisture change may occur. The footing system must suit the assessed class and site conditions.
Moderately reactive
Moderate movement can occur as clay or silt moisture changes. Drainage, landscaping and footing design become increasingly important.
Highly reactive
A higher range of characteristic surface movement is expected. The footing designer must account for the assessed movement and site features.
Highly reactive — greater movement than H1
H2 indicates a greater movement range than H1 and generally requires a more robust, project-specific footing response.
Extremely reactive
Extreme moisture-related ground movement may occur. Detailed engineering consideration and careful moisture management are important.
Problem or unusual site conditions
Used where one or more conditions require project-specific assessment, such as fill, soft soil, abnormal moisture, erosion, slope, mine subsidence or another factor preventing normal classification.
What does a Class P site mean?
Class P is often misunderstood. It does not automatically mean that a property is unsafe or cannot be developed. It signals that one or more “problem site” conditions require specific engineering consideration rather than relying only on the normal reactivity classes.
Examples can include uncontrolled or variable fill, soft soil, loose material, abnormal moisture conditions, erosion, slope instability, mine subsidence, collapsing soil or another condition that prevents a normal classification. The report should identify the relevant reason and explain what further investigation, design input or construction control may be needed.
Reactive clay in Sydney and Western Sydney
Reactive clay occurs in parts of the Sydney region, including areas of Western and South West Sydney, but geology and site history vary considerably. Nearby properties can have different fill depths, drainage, vegetation, weathered rock levels and moisture conditions.
It is therefore inaccurate to classify every property in a suburb as M, H1, H2 or P. A site classification should be based on field observations and subsurface information from the property, with laboratory testing where it materially informs the assessment.
What happens during a soil test for a house?
A proportionate residential site classification can include:
- Review of the proposed dwelling, available plans and site history
- Observations of slope, drainage, vegetation and existing development
- Subsurface investigation at accessible, representative locations
- Engineering description of soil, fill and weathered material
- Laboratory testing where it assists assessment of reactivity
- Consideration of abnormal moisture or other Class P features
- A written classification and information for the footing designer
Sydney Geotechnical’s site classification and residential soil testing service starts from $750 for a suitable standard scope. Existing structures, difficult access, complex ground conditions or additional design questions can require a different scope.
How the classification affects footing and slab design
As expected ground movement increases, the footing system generally needs to accommodate greater differential movement. The structural engineer considers the site class together with building geometry, loads, construction type, founding material and other site-specific factors.
Drainage and long-term moisture management also matter. Concentrated stormwater, leaking services, poorly managed garden watering or major vegetation changes can create uneven moisture conditions. A footing design cannot compensate for every uncontrolled site condition, so owners and builders should follow the report and designer’s construction and maintenance advice.
When a broader geotechnical investigation may be needed
An AS 2870 classification may not answer every project question. Deeper foundations, significant retaining walls, basements, slopes, unusual fill, groundwater, commercial loads or council requirements may call for a broader geotechnical investigation. Send the project documents so the scope can be matched to the actual design need.
Questions to ask when reading the report
- What site class has been assigned, and are any Class P factors identified?
- What investigation locations and depths support the conclusion?
- Were access or existing structures significant limitations?
- Does the report identify fill, groundwater, shallow rock or drainage concerns?
- What information must the structural engineer or footing designer use?
- Are construction inspections or additional investigation recommended?
Common questions
Site classification questions
What is the best site classification for building a house?+
A lower-reactivity class generally indicates less expected moisture-related ground movement, but a class is not a quality score. Footings must suit the actual classification, bearing conditions and project-specific site features.
Does a Class P site mean I cannot build?+
No. Class P identifies a site requiring project-specific consideration because of factors such as fill, soft soil, abnormal moisture, slope or other conditions. It does not automatically mean the land is unsuitable for development.
What is the difference between Class H1 and H2 soil?+
Both describe highly reactive sites. H2 indicates a greater range of characteristic surface movement than H1, so the footing designer must account for the assessed movement and other site conditions.
Are all Western Sydney properties reactive clay sites?+
No. Western Sydney contains varied geology, landforms, fill histories and drainage conditions. Site classification must be based on investigation of the particular property, not a suburb or postcode assumption.
Who uses a site classification report?+
The structural engineer or footing designer uses the classification and investigation information to design residential footings and slabs. Builders, certifiers and approval teams may also request the report.
Need a site classification in Sydney?
Get a scope matched to the project
Send the site address, proposed works, current plans and any request from your builder, engineer, council or certifier. Sydney Geotechnical will review the information and confirm an appropriate scope and fee.
This guide provides general information only and is not a substitute for investigation, engineering advice, footing design or approval advice for a particular property. Project requirements and ground conditions must be assessed site by site.