A soil report is a couple of pages that decide the most expensive concrete you will ever buy. Someone drills a hole or two in your block, tests the clay, and hands back a letter: A, S, M, H1, H2, E or P. Most people file it and never read it. The letter is the whole story, because it tells the engineer how much the ground under your house is going to move, and the more it moves the more steel and concrete you pay for.
The pro reads the classification line first, then the reactive movement number the report gives it, then whether the holes were drilled on your block or borrowed from next door. A class M on a real bore log is a number you can build to. A class M assumed off a neighbouring site is a guess wearing a suit.
The letter on a soil report comes from AS 2870, the residential slab and footing standard. It is not about how strong the ground is. It is about how much it swells and shrinks as it gets wet and dries out. Clay is the villain. Sand and rock barely move, so a house on them sits still. Reactive clay drinks water in winter and lifts, then dries out in summer and drops, and it does not do it evenly. One corner near a leaking downpipe stays wet and high, the corner under the sunny eave dries and sinks, and the slab between them tries to twist. That twist is what you see as a stair-step crack over a door or a gap opening at a cornice.
A lot of Adelaide sits on some of the most reactive clay in the country, and Melbourne's west and north are no better. That is why the soil class is the single line that moves the site cost more than any other on a flat, tidy looking block.
What is the reactive clay doing on my block through the year, and which side stays wettest?
The classes run from stable to problem. Each one is a band of expected surface movement, written in the report in millimetres. The bigger the movement, the deeper and stiffer the slab has to be to ride over it, and the more it costs. Here is what each letter actually means when you are standing on the block.
A and S are the easy ones. M is the workhorse class most volume base prices quietly assume. H1, H2 and E are where the money is, and P is the one that means "get an engineer, this is not a table job".
What class did the report return, and what movement in millimetres sits behind that letter?
The class picks the footing system. Class A and S get a standard waffle pod or a shallow stiffened raft, the cheapest way to pour a floor. Class M gets a deeper edge beam and more steel through the raft. H1 and H2 push the beams deeper again, add internal beams gridded across the slab, and thicken the steel, so the same floor area quietly uses a lot more concrete and reinforcement. Class E usually means a fully engineered slab, sometimes with piers driven down to stable ground so the house hangs off the deep material instead of floating on the moving clay. Class P is designed from scratch for whatever the problem is.
This is why a "site costs" line jumps when the report lands. The base price assumed class M. Your block came back H2. The difference is not the builder gouging you, it is a metre of extra beam depth and a tonne of extra steel that the standard slab never had.
What footing system has my slab been priced on, and does it match the class the report returned?
Flip to the bore logs or the site plan on the report and find where they tested. A report should have at least one borehole on your actual block, and two is better on anything bigger or sloping, so the reactivity is not read off one lucky spot. If the report says the class was "adopted" or "inferred" from an adjacent lot, it is an assumption, and the real class only turns up when the excavator does. On an estate that was cut and filled, the fill on your block can behave completely differently to the natural ground next door.
While you are in the logs, look for the depth to firm material and any note about fill, moisture or a water table. Deep fill or a high water table changes everything, and both are reasons a P classification appears on a block that looked flat and boring.
Were the boreholes drilled on my block, how many, and is the class measured or adopted from nearby?
The class on the day is not the class forever. A big tree near the house dries the clay on one side, so even a class M block behaves worse at that corner, and the engineer may add beams or a root barrier for it. Removing a large tree can be as bad, because the clay it kept dry slowly swells back over years and lifts that corner. Fill that was not compacted properly settles. A leaking pipe or a downpipe dumping against the slab keeps one edge wet while the rest dries. The report will often note these, and a good one recommends what to do about them.
This is the paragraph people skip and then ring a lawyer about in five years. Read the recommendations, not just the letter.
Do any trees, fill or drainage issues on my block change the design, and has the report allowed for them?
A soil report is easiest to fix before you own the land. Once you have signed, the class is your problem and the extra footing money comes out of your pocket. So if a report lands during a cooling-off period or before an unconditional contract, read it for the warning signs below and use them to get a better report, a price adjustment, or a walk-away. None of this needs an engineering degree. It is just knowing which lines a builder would refuse to accept.
The pattern to watch for is a report that sounds confident but never actually tested your block. A class letter with no borehole on your lot, no movement figure behind it, and no mention of the fill or the tree in the corner is a guess dressed up as a result. Here is what a weak report looks like, and the exact thing to say.
Was this block actually tested, and is every class, fill and tree issue answered in writing before I sign?
If any of these are on the report you were handed, do not sign until you have an answer. Say the line, get it in writing.
The class is "adopted" or "inferred" from a neighbouring lot, with no borehole shown on your block.
“The class here is adopted from next door. I need a bore drilled on this actual lot and a report for this lot before I go unconditional.”
Only one borehole on a big, sloping or irregular block.
“One hole is not enough to classify a block this size. Can we get a second bore so the class is not read off one lucky spot?”
The report is old, or the lot number and address do not match the land you are buying.
“This report is dated for a different lot. I need a current report that names my lot number and address.”
It is a cut-and-fill estate but the report says nothing about fill depth or compaction.
“This is a filled block. What is the fill depth, was it compacted to a certified level, and what class does the fill itself get?”
A class letter is given with no surface movement figure in millimetres behind it.
“What reactive movement in millimetres sits behind this class? I want the number, not just the letter.”
A large tree sits near the building envelope and the report does not mention it.
“There is a big tree inside the zone of influence of the slab. Has the classification allowed for it, or do we need a root barrier or deeper footings?”
A class P (problem site) is noted in passing with no design or cost attached.
“The report calls this a class P site. What does the engineered footing for a class P actually cost, and is that allowance in my building contract?”
The pages on BuildPilot this guide leans on.
They rank how much the ground swells and shrinks with moisture under AS 2870. A is stable sand or rock, S is slightly reactive, M is moderate, H1 and H2 are highly reactive clay, E is extreme, and P is a problem site (fill, softness, slope or trees) that needs site-specific engineering rather than a standard footing table.
Class M is the workhorse, and it is the class most advertised base prices quietly assume. Large parts of Adelaide and Melbourne's outer suburbs come back H1 or H2 once a real test is done, which is why site costs often rise after the report lands.
A lot. Moving from the assumed class M to H2 or E can add many thousands in extra concrete, steel and beam depth, and a class E or P site may need bored piers, which is a bigger step again. The exact figure depends on the house size and the engineer's design.
A residential soil test is typically a few hundred to around a thousand dollars, which is cheap next to redesigning footings later. If you are buying land, ask whether a test already exists and get a copy.
The class itself is fixed by the test, but what sits around it changes the design: a large tree, uncompacted fill, a downpipe dumping against the slab or a high water table can all make even a moderate class behave worse at one corner. Read the report's recommendations, not just the letter.
Make sure a borehole was actually drilled on your lot (not adopted from next door), that there is more than one hole on a big or sloping block, that the report names your lot and is current, that fill depth is addressed on a cut-and-fill estate, that a movement figure in millimetres sits behind the class, and that any large tree or class P note comes with a design and a cost. Get every answer in writing before you go unconditional.
Sizes quoted are typical for Australian residential work and are given to help you read drawings and ask better questions. They are not a specification. Your engineer, designer, builder and the National Construction Code decide what applies to your house.
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