In Australia, site classification follows the Australian Standard AS 2870, which sets out how residential slabs and footings should be designed. Each site is given a site class (A, S, M, H1, H2, E or P) depending on how reactive the soil conditions are. Understanding your site class early allows your builder to create a strong, compliant footing system that will protect your new home from structural damage such as cracking or movement.
Why Site Classification Matters Before You Build
Every home relies on a foundation that can provide structural support in line with its soil reactivity. Perth’s unique geography includes both sandy coastal areas and clay-based inland regions, which means ground movement can vary significantly from one suburb to another.
A professional classification gives engineers the data they need to select the right footing systems for your build. It prevents issues like wall cracks, slab lifting or uneven floors caused by soil movement or abnormal moisture conditions. It also ensures compliance with council requirements and the AS 2870 code.
WOW Homes manages this process early in your building process, streamlining your path from soil testing to final approvals. You can learn more about how soil is assessed in our article on the 3 Points of a Block Soil and Site Review in Perth.
How Site Classification Works in Australia
The Australian Standard – AS 2870
AS 2870 outlines the guidelines for residential slabs and footings. It defines how engineers measure characteristic surface movement, measured in ‘ys’, and design foundations that can handle that movement safely.
Soil Testing and Site Investigation
To determine a site class, engineers carry out several key steps:
- Soil testing: Taking various soil samples from different depths to analyse moisture content, texture and composition.
- Site investigation: Identifying soil types, soil properties, and site conditions including vegetation, slope and drainage.
- Borehole drilling: Measuring the crack zone depth and the profile of reactive materials.
- Laboratory testing: Calculating characteristic surface movement and evaluating moisture variation.
- Geotechnical report: Compiling results into a formal site classification report used by structural engineers.
These steps reveal whether your block consists of sandy soil, clayey soil, or soft soils like loose sands, each reacting differently under changing weather conditions.
What Influences Site Classification
Soil reactivity depends on several following factors:
- Moisture content: Changes in rainfall or irrigation can cause significant soil expansion or contraction.
- Vegetation: Trees near your block can create abnormal moisture conditions by drawing water from the soil.
- Drainage and slope: Poor drainage leads to uneven wetting, increasing the risk of soil movement.
- Fill and excavation: Uncontrolled fill or disturbed ground often results in unstable ground profiles.
- Climate and site history: Perth’s dry climates cause clays to shrink, while winter rains make them swell.
These factors highlight why one property may be classified differently from the one next door. For guidance on choosing the best location, read our article on up-and-coming Perth suburbs for first home buyers.
Common Misunderstandings About Site Classification
- “My neighbour’s site will have the same class as mine.” Soil conditions and moisture content can vary drastically, even on the same street.
- “Reactive clay means I can’t build.” With proper foundation design, you can build safely on most reactive clay sites.
- “Class P sites are unsafe.” They’re simply problem sites that need more investigation, not impossible builds.
When managed correctly, even extremely reactive clay or soft clay can support a lasting structure.
Additional Considerations for Perth Home Builders
Soil and Drainage Management
After construction, maintaining stable site conditions is vital. Keep gardens and trees positioned away from footings to reduce moisture changes that can cause soil movement.
Energy Efficiency and Insulation
Your foundation design can also impact energy performance. See our post on how important proper insulation is for advice on keeping your new home efficient and comfortable.
Block Orientation and Location
If you’re still planning your build, our guide on how to determine house orientation explains how land slope and sun direction influence design, drainage, and energy savings.
You can learn more about the construction phase in our article on the Slab Down Stage.
How WOW Homes Helps You Build with Confidence
At WOW Homes, we partner with qualified geotechnical engineers and structural engineers to assess your land thoroughly. From the first site investigation to final footing design, our team ensures your foundation meets the Australian Standard for residential slabs.
We handle all the details, including soil testing, engineering liaison, and approvals, so you can focus on the exciting parts like choosing your layout and finishes. If you’re a first-home buyer, explore our tips for first home builders in Perth to understand every step of the journey.
Frequently Asked Questions
What is the purpose of site classification?
It determines soil reactivity and guides engineers in selecting footing systems that resist movement and maintain structural integrity.
What are abnormal moisture conditions?
These occur when water levels around your home vary due to leaks, trees or poor drainage, altering the soil’s moisture balance and causing movement.
Can you build on reactive clay?
Yes, with reinforced foundation types such as stiffened raft foundations or waffle raft foundations.
How long does a site classification take?
Most reports are completed within one to two weeks, depending on testing requirements and site accessibility.
What happens if my site is classified as P?
Further testing is performed, and engineers design footing systems that safely anchor your home into stable ground.
Ready to Build with Confidence?
A professional site classification gives you clarity, confidence, and compliance. It ensures your foundation matches your soil conditions, protects your home from structural damage, and helps your builder make informed design decisions.
At WOW Homes, we make this process seamless by coordinating soil tests, working with project engineers, and delivering a foundation that lasts for generations. Ready to begin? Contact WOW Homes today to start your building journey with a team that understands Perth’s diverse soil types and how to build homes that stand the test of time.
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