A geotechnical report, often called a soil test or soil report, tells you what the ground under a house is made of and how much it will move. It gives the site a class, and the class drives the footing and slab design. Order it before you give a fixed price, or you are pricing the footings blind.
What a geotechnical report tells you
The report sorts the site into a class by how much the ground is likely to move as its moisture changes, and that class guides the footing and slab design. The National Construction Code’s Housing Provisions say foundations must be classified in accordance with AS 2870. In practice a geotechnical engineer takes samples across the footprint, tests them and assigns the class, and looks at how the ground varies across the block and at any planned cut and fill.
Report formats differ, but you should be able to find these things in it: the site class, the soil types found, any fill, the effect of trees, groundwater or abnormal moisture, and any comments on footing options.
Site class and what pushes a block to a harder class
The Housing Provisions give a general guide to the classes, in these terms:
| Class | What the National Construction Code says |
|---|---|
| A | Most sand and rock sites, little or no ground movement from moisture changes |
| S | Slightly reactive clay, only slight ground movement |
| M | Moderately reactive clay or silt, moderate ground movement |
| H | Highly reactive clay, high ground movement (engineers’ reports often split this into H1 and H2) |
| E | Extremely reactive clay, extreme ground movement |
| P | Problem sites: soft soils such as soft clay or silt or loose sands, landslip, mine subsidence, collapsing soils, soils subject to erosion, and reactive sites subject to abnormal moisture conditions |
Filled sites can fall in any class and are dealt with in AS 2870. The same page says that if a site is classified H, E or P, reference must be made to AS 2870 for design and construction, so expect an engineer’s footing design rather than a standard detail. That is what moves the cost.
AS 2870 is a paid Australian Standard and this article does not reproduce it. Read the class from your engineer’s report and ask them to explain anything you do not follow.
Who orders it and when
The job decides it. Often the homeowner or designer orders it for the approval. On a quote-first job, the builder may need it before a fixed price is sensible. Queensland’s building regulator tells owners comparing quotes that the builder must provide a soil test and contour survey, unless the owner already has them, to justify earthworks and footing costs, and notes reports of some builders charging excessively for them. So the regulator expects both to exist before quotes are compared.
Consumer Affairs Victoria goes further for builders. Its checklist for preparing a major domestic building contract says to obtain foundation data and a soil report, and that it is your responsibility to satisfy yourself that the foundation data is accurate. If you rely on a report, check the date, who it is addressed to and that it covers the footprint you are building on.
What it changes in the price
A harder class means a heavier footing or slab, more steel, and sometimes piers. Fill, trees and past demolition add questions. Rock or a high water table changes excavation. You cannot see any of that by walking the block.
The cost impact is real but depends on the site. We have not quoted figures here because we could not verify any from a regulator. Get the engineer’s recommendation and price from it.
Provisional sums are the usual shortcut when the ground is unknown (see provisional sums and prime cost items). Victoria’s checklist describes them as covering additional work where, after reasonable enquiries, you cannot give a definite price, says to state them clearly and make a reasonable allowance, and tells builders to specify as much as possible for a fixed total price to avoid disputes. A footing allowance that turns into a large variation is one of the most common ones. Order the report and you can price the footings properly.
What to say to the client who wants a price first
Keep it short. Something like this:
I can give you a rough range today. I cannot give you a fixed price for the footings until we know the soil. The report is what tells us the slab and footing type, and that is a big part of the number. If I guess low, you will get a variation later. If I guess high, I might lose the job for no reason. Let’s get the report and I will price it properly.
Most clients accept that when it is put as protection for them. The ones who will not often end up in a dispute over the footings.
Putting it inside a paid early stage
The soil report belongs in the same paid first stage as the site survey. Queensland’s regulator names soil tests and contour surveys as examples of early work a contractor can be paid for under a preliminary agreement, and warns that fees or penalties for a client who does not go ahead may be illegal. How that stage works, and what the regulators say about it, is in feature and contour surveys; the fee itself is covered in charging for preliminary work.
Whatever the stage, say in your agreement who owns the soil report and whether the client can take it to another builder. Our pre-construction page covers how we help builders run that stage.
This is general information, not engineering or legal advice. Rules differ by state: check with your regulator, engineer or a solicitor for your situation.
Common questions
What is a geotechnical report?
A report by a geotechnical engineer, based on soil samples and testing, that describes the ground on a site and gives it a class. The class guides the footing and slab design.
Is a soil test the same as a soil classification?
The soil test is the sampling and testing. The classification is the result: A, S, M, H1, H2, E or P. Reports usually contain both.
How much does a geotechnical report cost?
We have not quoted a figure because we could not verify one from a regulator. Get written quotes from local geotechnical firms and compare the scope, such as number of test points.
Can I quote a fixed price before the soil test?
You can, but you take the risk. If the ground is worse than you assumed, you absorb it or you run a variation. A clearly labelled allowance, or a report first, is safer.