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Site classification — the soil test that sets your slab cost

From Modular Living Australia — the independent knowledge base Last updated: 8 Oct 2026

Site classification under AS 2870 grades how much the soil moves into classes A, S, M, H1, H2, E and P, and the National Construction Code requires sites classed H, E or P to be designed by reference to AS 2870.^[1][2] Commercial guidance puts a site classification report at roughly A$400–A$1,200.^[3][4]

Almost every quote for a factory-built home in Australia contains a line item that cannot be priced when the quote is written. The site classification — a letter between A and P describing how much your soil moves when it gets wet and dry — determines the footing system, and the footing system is one of the largest single variables in a build budget.

The uncomfortable part of the sequence is that most buyers sign before the class is known.

What the classes mean §

Australian soils move. Reactive clays swell when wet and shrink when dry, and a footing that ignores that will crack the building above it. AS 2870, the residential slabs and footings standard, exists to match the footing to the expected movement, and the National Construction Code picks it up for Class 1 and Class 10 buildings^[1].

A class P site is not an unbuildable site. It means the standard designs do not fit and the footing has to be engineered specifically. Urban infill blocks with a history of fill frequently land here.

The line that costs money §

This is the provision most worth understanding, and it sits in the Code rather than in a builder's brochure.

The ABCB Housing Provisions contain deemed-to-satisfy footing designs — pre-engineered details you can build to without commissioning bespoke structural design. But those provisions have limits. Where a site is classified H, E or P, the Housing Provisions explicitly defer to AS 2870 for design and construction information^[1].

There is a second trigger that catches people in dry climates. For a Class M site, a further division applies based on the depth of expected movement. Where movement is deep-seated — a design depth of suction change of 3 m or more, characteristic of dry climates — the classification becomes M-D, and footing design then falls outside the scope of the Housing Provisions and must go back to AS 2870^[1].

The practical translation:

• A, S, M — standard designs available. Predictable.
• M-D, H1, H2, E, P — a structural engineer designs the footing to AS 2870^[1]. Engineering fee, longer lead time, and a heavier footing.

The gap between those two groups is where site cost blowouts come from. It is not that reactive sites are unbuildable. It is that a quote prepared assuming a standard raft slab and a site that turns out to be H2 are two different projects.

When the class is determined §

Classification comes from a geotechnical investigation of your specific block — boreholes, samples, and a report. Commercial providers describe the report as a prerequisite: the building surveyor requires it before footings can be designed or a building permit issued^[3].

Published cost guidance varies. One provider states roughly A$400–$800 for a residential block, with one to two weeks turnaround^[3]. Another states A$500–$1,200 excluding GST depending on the number of test holes, site access and location^[4]. Both are commercial guidance published by companies selling the service, not independent survey data, and the range is wide enough that the difference between the two is itself informative.

Against a build budget these are trivial figures. Against the consequences of guessing wrong, they are the cheapest money in the project.

Why this matters more for a factory-built home §

For a conventional builder, site classification and construction are handled by the same organisation, and the footing is designed as part of the same job. Off-site construction routinely separates them, and that separation is where the cost lands on the buyer.

Most modular quotes are for the modules. The slab or footing system is frequently a separate scope — sometimes a separate contractor entirely — and the base price assumes a benign site class. That assumption is rarely stated as an assumption.

Two things follow.

First, the fixed price is not fixed against the ground. A fixed-price modular contract can be entirely honest and still leave you exposed, because the fixed price covers the part of the project that happens in a factory, and the ground is not in the factory.

Second, the sequence works against you. Modular's speed advantage comes from footings being prepared while the modules are built. That only works if the classification is done early. A report commissioned late can hold up the entire programme, or worse, arrive after modules are complete and reveal that the footing design has to change.

The defensible approach is to get the soil report before signing, not after. If the land is already yours, there is nothing stopping you commissioning it independently — and a builder who discourages that is telling you something.

Questions worth asking before you sign §

• What site class does this quote assume?
• Who pays if the classification comes back worse than that?
• Is the footing system inside the fixed price, or a separate scope?
• Who commissions the geotechnical report, and who owns it?
• If an engineered footing is required, whose engineer, and at whose cost?

What is not covered here §

• Your site's class. It cannot be predicted from a neighbour's block, a map, or a postcode. Only a site-specific investigation determines it.
• Footing prices by class. No dollar figures for slabs or footings by site class are published here — the sources compiled here do not contain reliable independent figures, and builder-published ones vary too widely to be useful.
• The text of AS 2870. The standard is a copyright publication of Standards Australia and is not reproduced or summarised in detail here. This page describes how the Code refers to it.
• Which NCC edition applies in your state. States adopt Code editions on their own timetables. Confirm the current adopted edition with your certifier.
• Other site assessments. Wind classification, bushfire attack level and acid sulfate soil assessment are separate exercises, each covered elsewhere on this site or not at all.
• Contamination and remediation. Contaminated land is a separate regulatory process, not a site classification question.

References §