Your soil test tells you how much the ground under your house will move as it wets up and dries out, and it puts that answer into a site classification under AS 2870 Residential slabs and footings. The classes run A, S, M, H1, H2, E and P, from effectively stable to severely reactive or otherwise problematic. This is not optional paperwork: clause 4.2.2 of the ABCB Housing Provisions states that “the foundations where footings and slabs are to be located must be classified in accordance with AS 2870”. Your class then sets the footing design – beam depth, reinforcement, articulation, and whether an engineer has to design the system for your block specifically – which is why two otherwise identical houses on two different blocks can carry very different slab costs.
Why Queensland ground moves in the first place
Reactive soils are clays that swell when they take on water and shrink when they lose it. Sand and rock barely react; clay reacts a great deal, and south-east Queensland gives it exactly the conditions it needs to misbehave. The QBCC puts it plainly in its homeowner guidance on subsidence: “Dry seasons cause these soils to dry out rapidly. Wet seasons cause these soils to become wet quickly. This can cause sudden extreme volume changes and soil movement.” The same QBCC page notes that engineers in Queensland design footings “to the Australian Standard AS2870 – 2011 Residential slabs and footings”. A slab designed for the wrong class rarely fails straight away. It cracks in the third or fourth summer, after a full wet-dry cycle with a house sitting on it.
The AS 2870 site classification table
Below is what each class means, the characteristic surface movement (ys) band AS 2870 attaches to it, and what it does to your footing system. The descriptions in quotation marks come from explanatory Table 4.2.2 of the ABCB Housing Provisions. Two things to read carefully: that NCC table uses a single “H” row, while AS 2870 splits highly reactive sites into H1 and H2 by movement; and the cost column is directional only, because no honest builder prices a slab from a class letter alone.
| Class | Foundation description | Characteristic surface movement (ys) under AS 2870 | Typical slab and footing response | Direction of cost |
|---|---|---|---|---|
| A | NCC Table 4.2.2: “Most sand and rock sites with little or no ground movement from moisture changes” | Negligible | Standard stiffened raft or waffle raft built to the base details in AS 2870 | Baseline for reactivity – no reactive-soil premium on the footing system |
| S | NCC Table 4.2.2: “Slightly reactive clay sites with only slight ground movement from moisture changes” | Up to 20 mm | Stiffened raft, modest edge beam depth, standard reinforcement | Marginally above baseline |
| M | NCC Table 4.2.2: “Moderately reactive clay or silt sites which can experience moderate ground movement from moisture changes” | Over 20 mm and up to 40 mm | Deeper edge beams, heavier reinforcement, internal beams on a tighter grid | A clear step up in concrete, steel and excavation |
| M-D | AS 2870 adds a “-D” suffix (M-D, H1-D, H2-D, E-D) where the moisture change is deep-seated rather than shallow | Same band as M, but designed for a deeper depth of suction change | M detailing carried deeper, designed for movement well below the surface | At or above M |
| H1 | NCC Table 4.2.2 covers both H classes in one row: “Highly reactive clay sites which can experience high ground movement from moisture changes” | Over 40 mm and up to 60 mm | Engineered raft. The simplified Housing Provisions pathway stops at Class M | Step change. Site-specific engineering becomes unavoidable |
| H2 | Same NCC description as H1; AS 2870 splits the highly reactive band at 60 mm | Over 60 mm and up to 75 mm | Stiff engineered raft, often with bored piers or screw piles, plus articulated masonry | Above H1 |
| E | NCC Table 4.2.2: “Extremely reactive clay sites which can experience extreme ground movement from moisture changes” | Over 75 mm | Fully site-specific system: deep piering to a stable founding layer, very stiff slab, full articulation | The most demanding of the reactive classes |
| P | NCC Table 4.2.2: “Sites which include soft soils, such as soft clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soils subject to erosion; reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise” | Not expressed as a ys band. Classified on the problem, not the reactivity | Geotechnical investigation first, then an engineered fix: piling, soil replacement, surcharging or retention | Open-ended until the engineer has scoped the remedy |
Two things in that table matter more than the numbers. First, the simplified code details only carry you across A, S and M. The Housing Provisions note under Table 4.2.2 says that “due to the limitations of this Part, if a site is classified H, E or P then reference must be made to AS 2870 for design and construction information”, and NCC Volume Two clause H1D4 only lets you use that Housing Provisions pathway where the “footing is on a Class A, S or M site (classified in accordance with AS 2870) with a uniform bearing capacity”. Above Class M you are in AS 2870 proper, with an engineer designing for your block.
Second, fill sits outside the neat progression. Table 4.2.2 gives filled sites the class range “A to P” and the single instruction “Filled sites – see AS 2870”, which is the code’s way of saying that uncontrolled fill left by a previous owner can drag an ordinary-looking block anywhere on the scale.
What actually drives the cost difference
People assume the jump from S to H1 is a materials cost. Partly, but not mostly. The real drivers are:
- Concrete and reinforcement. Deeper, more frequent stiffening beams mean more excavation, concrete and steel across the same floor area.
- Engineering input. On A, S and M sites a slab can often be detailed from standard tables. From H1 upwards the NCC sends you to AS 2870, so an engineer designs and certifies for your block specifically.
- Piering. Bored piers or screw piles to a competent founding layer are a separate trade, machine and set of days on site.
- Everything the slab touches. Reactive sites need articulated brickwork, control joints, flexible plumbing connections and disciplined drainage, so the class quietly reprices other trades.
- Interaction with slope. A reactive class on a steep block compounds rather than adds, because cut, fill and retaining all change the moisture regime – see building on a sloping block in Queensland.
For where footings sit inside a whole-of-project budget, see the cost to build a house on the Sunshine Coast.
Class P is a referral, not a verdict
A P does not mean the block is unbuildable. It means AS 2870’s standard designs cannot simply be applied and a geotechnical engineer has to establish what is happening below the surface. Plenty of very good Sunshine Coast and Noosa homes sit on P sites – on piles, on replaced soil, or on ground that was surcharged and re-tested. What it does mean is that you should not lock in a fixed price before the engineered footing design exists, because footings are the item most capable of moving by a large amount.
Who arranges the soil test, and when
Get it early. The classification is what your architect and engineer design around, so ordering it after the plans are drawn is how owners end up reworking a floor plan to suit a footing system.
Queensland contract law pushes the same way. The QBCC’s guide to domestic building contracts explains that section 31(2) of Schedule 1B of the QBCC Act “requires contractors to obtain any foundations data necessary for the work before they enter the building contract”, and that this preliminary work, “especially tests to obtain foundations data”, will “often be essential to enable the builder to calculate an accurate price for the building contract”. The QBCC also notes that obtaining foundations data – “soil tests and contour surveys to determine the soil classification and earthwork costs” – falls under the definition of excluded building work in section 1 of Schedule 1B, which is why it is commonly covered by a preliminary agreement signed before the main contract. A builder pricing your slab without a classification is guessing, and the guess gets corrected later as a variation.
On our new home builds across the Sunshine Coast the geotechnical investigation runs alongside the survey. Because clause 4.2.2 requires the foundations to be classified in accordance with AS 2870 before a compliant footing design exists, the classification has to be in hand before the engineering is finished and lodged for approval.
What we see across Sunshine Coast and Noosa blocks
You cannot read a classification off a postcode, but patterns repeat across our service area. In the coastal sand belts – Coolum Beach, Yaroomba, Marcus Beach, Noosa – deep marine sand often classifies benignly, but shallow water tables, buried organic layers and coffee rock appear often enough that the bore log matters more than the class letter. On the river and creek flats around Cotton Tree, soft alluvial silts are where soft-soil P classifications turn up and where settlement, not shrink-swell, governs. On hinterland slopes such as Ninderry, weathered volcanic profiles produce genuinely reactive clays. And on any previously worked block, old uncontrolled fill is the most common reason a straightforward-looking site comes back worse than its neighbours.
Council overlays sit alongside the soil test, so check both before you commit to a footing strategy. Part 8 of the Sunshine Coast Planning Scheme 2014 contains 13 overlay codes, including the Acid sulfate soils overlay code (8.2.1) and the Landslide hazard and steep land overlay code (8.2.10). The acid sulfate code aims to avoid releasing acid and metal contaminants by “not disturbing acid sulfate soils (ASS) when excavating or otherwise removing soil or sediment, extracting groundwater or filling land”, and its overlay maps identify “Area 1 (land at or below 5 metres AHD)” and “Area 2 (land above 5 metres AHD and below 20 metres AHD)” – which is precisely the sort of work deep beams, piering and site cut involve. In Noosa Shire the scheme is The Noosa Plan 2020, which applies overlays over the top of the zone, so look your lot up in Noosa Shire Council’s Noosa Plan 2020 mapping before you assume the soil report is the only constraint on your earthworks.
Keeping the slab in the class it was designed for
A footing designed for an H1 site assumes moisture conditions around it stay within a normal range, and you can push a site outside that range after handover. The QBCC advises homeowners to “keep the site well drained by avoiding: altering the site drainage, placing gardens, garden edging or gravel pavements next to your house”. It also warns that “planting a tree next to the building can take up a lot of ground water and cause subsidence when fully grown”, and that “cutting down a well-established tree next to your home could create a subsidence problem”.
On cracking, the QBCC points to AS 2870 and treats walls with hairline cracks, and “walls with cracks that are noticeable but easily filled and are less than 5mm wide”, as acceptable under the standard. If damage goes past that, tell your builder – the QBCC states that builders are “responsible for fixing any issues caused by their building work within 6 years and 6 months of them commencing the work”. Before you start, confirm the licence: how to check a QBCC licence.
Frequently asked questions
Who carries out a soil test and who classifies the site?
A geotechnical consultant does the boreholes and laboratory work and issues a report containing the AS 2870 classification. Your structural engineer then designs the footing system to that class. The Housing Provisions require the foundations to be classified in accordance with AS 2870, so it is not something a builder assigns by eye on a site walk.
Can my site classification change?
Yes. Cutting, filling, altering drainage, adding a pool or removing large trees all change the moisture regime the original classification assumed. Significant earthworks generally warrant re-testing rather than relying on the earlier report.
Does a Class A site guarantee a cheap slab?
No. Class A speaks only to reactivity. Bearing capacity, water table depth, slope, flood immunity, fill and coastal exposure all still apply, and any one of them can drive footing cost up on a Class A block.
What is the difference between H1 and H2?
Both are highly reactive clay, and the NCC’s explanatory table describes them in a single row. AS 2870 splits the band by characteristic surface movement: H1 covers movement over 40 mm and up to 60 mm, H2 covers over 60 mm and up to 75 mm. In practice H2 usually means stiffer detailing and more piering.
Should the soil test be done before I sign a building contract?
Before, wherever possible. The QBCC’s guide to domestic building contracts is explicit that section 31(2) of Schedule 1B requires contractors to obtain the foundations data they need before entering the building contract, and pricing footings from a real classification rather than a provisional allowance is the difference between a contract that holds and one that generates variations.