Class P site classification: what "problem site" really means for your build
You've got your soil report back, you're flicking to the classification line, and instead of an M or an H you see a single letter you weren't expecting: P.
Class P stands for "problem site", and it's the one classification that makes builders pause. It usually means the standard slab designs don't apply, an engineer has to get involved, and the footing cost is about to climb. Let's unpack what actually triggers a Class P, what it means for your project, and how to spot the risk before you've committed.
What the classes mean
Under AS 2870, the standard that governs residential slabs and footings across Australia, every new home site gets a classification based on how much the ground is likely to move as it dries out and wets up.
The scale runs from stable to reactive:
- A – stable, little or no movement (sand, rock)
- S – slightly reactive clay
- M – moderately reactive
- H1 / H2 – highly reactive
- E – extremely reactive
- P – problem site
A to E are about how much a clay swells and shrinks. P is different. A Class P site is one where the normal rules don't safely cover what's going on, so a standard slab design can't just be picked off the shelf.
What triggers a Class P classification
A site gets classified P not because the clay is necessarily worse, but because something about the ground needs engineering judgement rather than a standard design. Common triggers include:
- Uncontrolled or deep fill. If the block has been filled at some point and that fill wasn't placed and compacted to a documented standard, it can't be relied on to support a footing. Very common on cut-and-fill estates and older subdivisions.
- Soft or compressible soils. Soft clays, silts, or organic material that compress under load and cause settlement.
- Abnormal moisture conditions. Sites near drainage lines, with poor surface drainage, or where trees have dried out the soil unevenly.
- Sloping sites and cut-and-fill platforms. Where part of the slab sits on natural ground and part on fill, the two can move differently.
- Mine subsidence, reactive sites with extreme movement, or sites affected by erosion or instability.
In short, if the assessor can't confidently slot your block into A through E, it becomes P, and the footing system has to be designed specifically for that site.
What it means for your build
A Class P isn't a disaster, but it changes a few things.
The slab gets engineered, not standardised. Instead of a builder using a standard waffle or stiffened raft, a structural engineer designs a footing for that specific site. That might mean a deeper raft, screw piles or bored piers down to stable ground, or removing and re-compacting the fill.
The cost usually goes up. Piering, extra excavation, or removing and replacing poor fill all add to the slab cost, sometimes significantly. The exact figure depends entirely on what's causing the P, so it's worth getting the reason understood, not just the letter.
Timelines can stretch. Engineered footings mean more design time, and sometimes additional site investigation – deeper boreholes or test pits – before anyone can commit to a design.
The key thing: a Class P with a clear, simple cause (like shallow controlled fill) might be a minor cost. A Class P caused by deep uncontrolled fill or soft soils can be a major one. The letter alone doesn't tell you which.
Why fill is the most common culprit in QLD and NSW
Across South East Queensland and the Sydney and Hunter growth corridors, a lot of newer land has been earthworked into shape – hills cut down, gullies filled in, level pads created for easy building. That's great for the look of a block, but if the fill in the low side of a cut-and-fill lot wasn't certified, it can push an otherwise ordinary site into Class P.
This is why two neighbouring blocks on the same street can come back with very different classifications. One sits on natural ground; the other sits on three metres of fill over an old gully.
Desktop clues that a site might carry fill or movement risk: a block that was clearly levelled, proximity to a drainage line or watercourse, steep original terrain, or known reactive geology in the area. None of these confirm a Class P, but they tell you to budget for the possibility and order your soil test early.
How to protect yourself
The single best move is to get the soil classification done before you're locked in – ideally as a condition of your contract or before you sign a fixed-price build. A surprise Class P discovered after you've signed a standard slab allowance is exactly how budgets blow out.
And before you even order the soil test, a desktop look at the site's geology, slope, watercourses and fill history tells you whether you're in higher-risk territory. If the early signs point to soft ground or fill, you walk in expecting an engineered footing instead of being blindsided by it.
Common questions
Does a Class P site mean I can't build? No. It means the footing has to be specifically engineered for the site rather than using a standard design. Almost any block can be built on; a Class P just changes how, and usually how much.
How much more does a Class P slab cost? It depends entirely on the cause. A minor fill issue might add little, while deep piering or removing and replacing poor fill can add a substantial amount. Get your engineer to explain the reason for the P, because that drives the cost – not the letter itself.
Can I find out the risk before paying for a soil test? You can't get the formal classification without an on-site investigation, but a desktop review of geology, slope, watercourses and likely fill history will flag whether a site is higher risk – enough to budget sensibly and order the test early.
Want to know whether a block is sitting on reactive ground, fill, or near a watercourse before you commit? Pop in your address or check out a sample report and see what the ground is likely to throw at you.
LayeredGeo pulls together public planning, geology and site data for property due diligence across Queensland and New South Wales, so you can understand a place before you commit.