Why a site comes out Class P
AS 2870 gives Class P to any site where the ordinary A to E classes do not describe the risk. In practice, around Sydney and regional NSW, the usual reasons are these.
- Fill: soil placed by people, often in an old gully or to level a block, which can settle unevenly under a footing
- Soft or loose natural soil that could settle, such as alluvium along a creek or river flat
- A slope, where footings may need to be taken deeper or the site assessed for stability
- Trees close to the footings, which dry the clay unevenly and cause differential movement
- Abnormal moisture conditions, such as a site that has been under a house, a dam or a leaking service for years
- Ground that cannot be classified from the boreholes alone, such as a site over a mine or a site that flooded
What it means for the footing
For an A to E site the designer looks the footing up: AS 2870 gives deemed-to-comply slabs and strip footings for each class. For a P site the designer has to engineer it, using what the report says about the ground. That might mean a stiffer slab, piers or piles through fill to natural ground, deeper edge beams on a slope, or a footing set away from a tree's root zone. The report tells the designer what the problem is, where it is, and what the ground underneath it will support.
The house still gets built. The difference is that the footing is designed for your block rather than for a typical one, which is arguably a better outcome for something you will live in for decades.
What the report gives you
A good Class P report does not stop at the letter. It says why the site is P, how deep the fill or soft ground goes, what the natural ground beneath it is and how strong it is, and where the problem is on the block, so a designer can work around it rather than assume the worst everywhere. If the site is P because of a slope or deep fill, the report says whether a full geotechnical investigation is needed before the design can proceed, and we quote that if so.
Can you avoid a Class P result?
Not by choosing a different tester. The ground is what it is, and a classification that ignored fill or a slope would leave you with a footing designed for the wrong site. What you can do is get the classification early, before the design is finished, so a P result changes the footing on paper rather than on site. And if you know the block was filled or has a big tree near the footprint, tell us when you book: the boreholes can go where they will tell the designer the most.
Questions
Last checked 9 September 2026. Written by the engineers at Australian Ground Sciences, who prepare and sign the reports.