A site classification (also called a lot classification) is a geotechnical assessment that determines the soil characteristics of a residential building site in accordance with AS 2870-2011 — the Australian Standard for Residential Slabs and Footings.
A site classification is a mandatory requirement for nearly all residential building approvals across Australia.
What Is Site Classification?
Site classification is the process of evaluating a property's soil profile to determine how the ground will behave under a proposed structure. The classification directly informs the structural engineer's footing and slab design, ensuring the foundation can withstand ground movement without causing damage to the building.
Without a compliant site classification report, council Development Applications (DA) cannot proceed, and no structural engineer can certify the foundation design.
AS 2870 Site Classes
AS 2870 defines six primary site classes based on the expected magnitude of ground movement (primarily from reactive clay soils):
| Class | Classification | Expected Surface Movement | Typical Soil Type |
|---|---|---|---|
| A | Sand or rock sites | 0 mm | Sand, sandstone, granite |
| S | Slightly reactive clay | 0–20 mm | Low plasticity clay |
| M | Moderately reactive clay | 20–40 mm | Moderate plasticity clay |
| H1 | Highly reactive clay | 40–60 mm | High plasticity clay |
| H2 | Highly reactive clay | 60–75 mm | Very high plasticity clay |
| E | Extremely reactive clay | >75 mm | Extremely reactive clay |
| P | Problem site | Varies | Fill, soft soil, slope, mine subsidence, etc. |
P Class Sites
A P classification is assigned to sites identified as problem sites. This includes:
- Uncontrolled or poorly controlled fill
- Soft or collapsing soils
- Abnormal moisture conditions
- Landfill or contaminated ground
- Mine subsidence areas
- Sloping sites (>11 degrees often triggers slope stability requirement)
- Sites with trees causing abnormal moisture changes
A P classification means the structural engineer must provide a specifically engineered foundation design beyond standard AS 2870 footing details.
What's Included in a Standard Site Classification?
A compliant AS 2870 site classification report typically includes:
- Borehole drilling — 2 boreholes to 3m depth (standard) or up to 6m (custom)
- Soil profiling — identification of soil layers, fill, rock, and water table
- Laboratory testing — Atterberg limits (plasticity index), shrink/swell testing
- DCP testing — Dynamic Cone Penetrometer to measure soil density and strength
- Site classification — determination of AS 2870 class (A, S, M, H1, H2, E, or P)
- Wind classification — site wind rating per AS 4055 (N1–N6, C1–C4)
- Foundation recommendations — slab type and footing details for the engineer
- Additional observations — site drainage, tree locations, slope, fill presence
The Site Classification Process
Step 1: Desktop Study
Review existing geological maps, council records, and nearby site data.
Step 2: Site Visit & Drilling
A technician arrives with a drill rig and collects soil samples from boreholes (typically 2 holes to 3m depth). DCP tests are conducted in each borehole.
Step 3: Laboratory Testing
Soil samples are tested in a NATA-accredited laboratory for:
- Moisture content (AS 1289.2.1.1)
- Atterberg limits — liquid limit, plastic limit, plasticity index (AS 1289.3.1.1, 3.2.1, 3.3.1)
- Linear shrinkage (AS 1289.3.4.1)
- Shrink/swell index — for reactive clays (AS 1289.7.1.1)
Step 4: Engineering Assessment
A geotechnical engineer interprets the data and assigns the site classification, along with foundation recommendations.
Step 5: Report Delivery
A completed AS 2870 compliant report is issued, ready for the structural engineer and council submission.
When Do You Need a Site Classification?
- New home construction — required by the National Construction Code (NCC)
- Home extensions and additions — where foundations are involved
- Townhouse and duplex developments — multi-residential sites
- DA approvals — councils require it as part of the development application
- Before slab construction — confirms the footing design is appropriate
Common Questions
How long does a site classification take?
Standard turnaround is 7–10 business days for metro areas, including drilling and lab testing. Some providers offer faster service with in-house NATA-accredited labs.
Do I need to be on site?
No. The technician can access most sites independently. The report is emailed to you ready to hand to any engineer.
What does a site classification cost?
Costs vary by location, site complexity, and geotechnical provider. A standard residential site classification typically ranges from $600–$1,200 depending on location and inclusions.
Is shrink/swell testing included?
For reactive clay sites, shrink/swell testing should be included. Some providers offer this at no extra cost as part of the standard report.
Australian Standards
| Standard | Purpose |
|---|---|
| AS 2870-2011 | Residential slabs and footings — site classification, design, and construction |
| AS 1726-2017 | Geotechnical site investigations — field and laboratory methods |
| AS 4055 | Wind loads for housing — wind classification |
| AS 1289 Series | Soil testing methods (30+ individual standards) |
| NCC (National Construction Code) | Building code requirements for site classification |