Site classification and slab cost: Class M, H1, H2 and P

The AS 2870 site classification sets which footing design a house can use. Moving from Class M to Class H1, H2 or P changes beam depth, reinforcement, concrete volume and sometimes the footing system itself, which is why the site works figure in a building contract moves when the soil test comes back.

This piece sets out what each class means, what changes in the slab as the class rises, what puts a site into Class P, and what can be established about a lot before anyone drills.

What the class letter measures

AS 2870 classifies a residential site by its characteristic surface movement (ys), the amount the ground surface is expected to rise and fall through one dry-to-wet cycle. Lochaden and Haberfield (2018) set out the bands used for reactive clay sites:

Class	Characteristic surface movement (ys)	Reactivity
S	up to 20 mm	slightly reactive
M	over 20 mm to 40 mm	moderately reactive
H1	over 40 mm to 60 mm	highly reactive
H2	over 60 mm to 75 mm	highly reactive
E	over 75 mm	extremely reactive

Classes A (stable, non-reactive) and P (problem site) sit outside that sequence. A is sand or rock with little movement. P is a site the standard designs do not cover.

Two points follow from the table. First, the bands are narrow: a few millimetres of predicted movement can move a site between M and H1, or H1 and H2. Second, the classification describes the ground, not the house. It is an input to footing design, not the design itself.

What changes in the slab as the class rises

AS 2870 provides standard designs for Class S through H2 (Lochaden and Haberfield 2018). As the class rises, the deemed-to-comply design responds with deeper and more closely spaced stiffening beams, more reinforcement and a heavier slab. The practical effects on a build are:

• Concrete and steel quantities rise, so the slab line item rises with them.
• Excavation depth for the beams rises, which matters most where rock or fill sits shallow.
• The choice of footing system narrows. A waffle raft founds its beams near ground level with polystyrene void formers and involves little excavation (Lochaden and Haberfield 2018), which suits some sites and not others.
• Above Class H2, standard designs stop. Class E and Class P sites are engineered specifically.

Neither the standard nor this article can give a dollar figure for that step, because it depends on floor area, plan shape and local rates. Our breakdown of geotechnical report and soil test costs covers what the investigation itself costs at each tier.

What triggers Class P

Class P means the site falls outside the standard designs, so a geotechnical or structural engineer designs the footing for that lot from first principles. The triggers include soft or unstable soil, uncontrolled fill, landslip, mine subsidence, cut-and-fill platforms and severe abnormal moisture conditions. We set out the full list in Class P site classification.

Fill is the trigger most often decided by earthworks rather than geology. Delaney (2005) notes that the standard tolerates only a limited depth of uncontrolled fill, in the order of 400 mm, before Class P is warranted, and that the allowance reflects the assumption that ordinary slab footing excavations reach about that depth. The same paper observes how readily subdivision earthworks reach that thickness when topsoil, mulch and unsuitable material are spread across a lot.

The consequence for a builder or owner is procedural: a lot with unrecorded fill is not priced as a Class M lot with a contingency. It is priced as a site that may need an engineered footing, removal of the fill, or both.

What a compliant footing is designed to deliver

A footing built to AS 2870 is designed to limit distortion, not to prevent movement. The Foundation and Footing Society Victoria (2022) practice note states that footing systems built to the standard are an acknowledged compromise between cost and performance, that the standard defines expected performance through its own damage categories, and that Category 0 to 1 is the usual limit, with Category 2 accepted under adverse conditions.

Cameron (2018) makes the same point from the standard's side: complying designs may still show some movement and distress, usually cosmetic, and a client who wants a higher standard of performance can request an upgraded footing design at additional cost. That conversation is cheaper before contract than after the first dry summer.

Delaney (2005) reported that a review of one major insurer's files after the 1989 Newcastle earthquake found reactive soils were the primary cause behind 51 per cent of claims, and separately noted that damage is rare where the correct site class was adopted and severe moisture conditions, such as large trees close to the building, were avoided. The recurring failure is an optimistic classification paired with a site that later gets wetter or drier than the design assumed.

What can be established before the soil test

A classification requires a site investigation with sampling and laboratory testing. Before that, the following can be established from mapped and historical records, and each of them narrows the expected range:

• The mapped geological unit under the lot, which sets the plausible reactivity range.
• The AS 2870 climate zone, which sets the design depth of seasonal moisture change (Hs) and so scales the movement.
• Fill history, from historical aerial imagery, contours and terrain: filled gullies, dams, pads and batters.
• Nearby borehole logs: depth to clay, depth to refusal, standing water levels, and how much adjacent logs disagree.
• Slope and fall across the building platform, which drives cut, fill, retaining and drainage.
• Trees on or beside the lot that are staying, since they are a design input under the standard.

That evidence does not produce a class letter. It produces a defensible expected range to price against and a drilling brief that answers the right questions.

Common questions

What is a P class slab?

There is no standard P class slab. Class P means the site sits outside the standard designs in AS 2870, so an engineer designs a footing specifically for it. The result may be a stiffened raft, piers to a competent layer, or removal and replacement of the offending material.

What is an H1 class slab?

H1 is a highly reactive site with characteristic surface movement between 40 mm and 60 mm. AS 2870 provides standard raft and strip footing designs for it, which are deeper and more heavily reinforced than the equivalent Class M design.

Does the site classification change the cost of a slab?

Yes. A higher class means more concrete, more steel and deeper beams, and Class P removes the standard design altogether. The size of the difference depends on floor area, plan shape and local rates, not on the letter alone.

Can the site class be known before the soil test?

No. Only a site investigation by a geotechnical engineer produces a classification a certifier will accept. Mapped geology, climate zone, terrain, fill history and nearby bore logs give an expected range, which is enough to price and to brief the driller.

To see the geology, terrain, fill history and nearby bore records for a lot before the investigation is booked, generate a desktop report for the address, or see a sample report for what it contains.

LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland, New South Wales or Victoria.

Sources

• Andrew L.E. Lochaden and Chris M. Haberfield (2018). Assessment of the AS2870 standard designs for residential rafts on reactive clay. Australian Geomechanics, Volume 53, Number 1 (Mar 2018). geomechanics.org.au
• M. G. Delaney (2005). An overview of engineering geology and geotechnical challenges in the Newcastle Region. Australian Geomechanics, Volume 40, Number 1 (Mar 2005). geomechanics.org.au
• Donald A. Cameron (2018). Dealing with reactive clay soils through a national standard. Australian Geomechanics, Volume 53, Number 1 (Mar 2018). geomechanics.org.au
• Foundation and Footing Society Victoria Inc. (2022). Practice Note No. 8. Australian Geomechanics, Volume 57, Number 2 (Jun 2022). geomechanics.org.au

About this article. Published by LayeredGeo and written from the published research cited in the Sources section above. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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