Soil Tests and Site Classification for New Homes in Tasmania

A soil test tells you what classification your land gets under AS 2870-2011 — the Australian Standard that governs residential slab and footing design. That classification determines how complex (and costly) your foundation will be. For many Tasmanian blocks, the difference between a Class A site and a Class H2 site is $20,000–$50,000 in foundation costs. Here's what to expect from the process and why it matters before you sign a building contract.

What Is a Soil Test and Why Does It Matter?

A soil test — formally a geotechnical investigation — assesses the physical and chemical properties of your land to determine how it will behave under a building load. The results are used to classify the site under AS 2870-2011, the Australian Standard for residential slabs and footings. Every new home in Tasmania must have its footing design based on this classification.

Without a soil test, a builder can only estimate the site class — and that estimate will typically appear as a provisional sum in your contract. If the actual soil classification comes back worse than assumed, the provisional sum is adjusted upward. For some sites, that adjustment is significant. Getting a soil test done before you finalise your building contract gives you the actual number, not an estimate.

Tasmania has varied geology. The north-west and Launceston Basin areas include expansive clay soils that rate H1 or H2 under the standard. Coastal and river floodplain sites may rate E (erodible) or P (problem). Dolerite and basalt terrain in the Midlands and Central Plateau tends to produce better-performing Class S or Class M sites. Your postcode tells you something — but only a test tells you for certain.

The AS 2870-2011 Site Classes Explained

AS 2870-2011 defines seven site classifications. These determine what type of footing system your home requires — from a standard stiffened raft slab to a fully engineered deep pier system.

Most suburban blocks in established Tasmanian towns fall in the S–H1 range. Class H2 is common in some north-west coastal areas where clay soils are deep and expansive. Class P applies to filled land, reclaimed sites, and some older residential subdivisions where ground conditions have been disturbed.

What Happens During a Soil Test

A standard residential soil test is carried out by a geotechnical engineer. For a suburban block, the process typically involves:

• Site visit and borehole drilling: The engineer drills 2–4 boreholes across the site, usually to a depth of 3–4 metres. For flat suburban sites, this is done with a small rig that fits through a standard gate. Steep or remote sites may require different access equipment.
• Sample collection and laboratory testing: Soil samples are collected at intervals and tested for plasticity index (PI), linear shrinkage (LS), and moisture content — the key indicators of clay reactivity under AS 2870-2011.
• Written report: The engineer produces a report classifying the site under AS 2870-2011 and recommending a suitable footing system. This report goes to your building designer and structural engineer as a key design input.

Turnaround from site visit to report is typically 1–2 weeks for standard tests. Complex sites or those requiring deeper investigation may take longer. In northern Tasmania, geotechnical engineering firms include A1 Soil Testing (Launceston) and several Hobart-based firms that service the broader state.

How Much Does a Soil Test Cost in Tasmania?

For a standard suburban residential block in northern Tasmania, expect to pay $400–$900 for a basic soil test including report. More complex sites — sloping blocks, remote locations, sites with unusual soil profiles, or those requiring additional boreholes — typically cost $800–$1,500 or more.

These costs should be weighed against what the test protects you from. A provisional sum for foundations in a building contract might be set at $15,000 for a Class M assumption. If the site comes back Class H2, the actual cost can be $40,000–$60,000 — a difference that the soil test would have made visible before the contract was signed.

For context on how provisional sums work in a Tasmanian building contract, see our guide to provisional sums and prime cost items.

When to Get a Soil Test Done

The best time to get a soil test is before you finalise your building contract — ideally at the land assessment stage or during design development. This gives your designer and builder accurate information to work with from the start, rather than building in a provisional allowance that may change.

If you're purchasing land, a soil test can be a useful due diligence step alongside a feature and level survey. Some land agents will provide a soil test report for new estates — check whether one has been done before commissioning a new one. In subdivisions where cut and fill has occurred, the geotechnical conditions can vary significantly across what appears to be a uniform site.

For more on the land assessment process in Tasmania, see our guide to choosing land in Tasmania.

How Soil Class Affects Your Building Design

The soil classification directly determines the footing specification your structural engineer must design to. The progression from Class A to Class H2 requires progressively more steel, deeper beams, closer beam spacing, and in some cases deep bored piers rather than a slab-on-ground system.

Beyond the foundation itself, a high soil reactivity classification can influence other decisions: stump heights for subfloor areas, drainage design around the perimeter of the slab, and landscaping specifications (some engineers specify minimum distances from the slab edge for tree planting on highly reactive sites).

On sloping sites, a Class P classification may indicate that a pier-and-bearer system (subfloor construction) is a better solution than a complex cut slab — which can affect the whole architectural approach to the home. These decisions are best made early, when they can influence the design rather than require expensive modifications to an already-developed plan.

To understand how site classification and other factors affect the overall cost of building in Tasmania, see our guide to site costs and what affects build price.

Frequently Asked Questions

Is a soil test required to build in Tasmania?

A soil classification is required to design an AS 2870-compliant footing system, which is mandatory for residential construction. While the building permit process doesn't always explicitly require you to lodge a soil test report, your building surveyor and structural engineer will need the classification to certify the design. In practice, a soil test is a necessary step for any new home build on a residential block.

Can my builder determine the soil class without a test?

A builder can make an informed assumption based on local knowledge — neighbouring properties, council records, or a visual inspection of the site. But an assumption is not a classification under AS 2870-2011. If a builder quotes you a footing cost based on an assumed class and the actual test comes back worse, the difference is typically a contract variation. A soil test eliminates that risk.

What's the difference between a soil test and a site survey?

A soil test is a geotechnical assessment — it tells you about the ground's physical properties and how it will behave under load. A site (feature and level) survey is a spatial measurement — it maps what's on and around the land: contours, elevations, existing structures, services, and trees. Both are typically needed before design begins. They are done by different professionals: a geotechnical engineer for the soil test, a registered land surveyor for the feature and level survey.

Does the soil class change if I cut and fill the site?

If significant cut and fill is proposed, your engineer may recommend that the soil test be done after earthworks are complete — or that the geotechnical report address both the cut area (where the original profile remains) and the fill area (which behaves differently and typically requires engineered fill specifications). Fill material that's not properly compacted and tested can result in a Class P classification even if the underlying native soil is Class S or M. Your geotechnical engineer can advise on the right sequencing for your site.

How do I find a geotechnical engineer in Tasmania?

Several engineering firms offer residential geotechnical services in Tasmania, including firms based in Launceston and Hobart that service regional areas. Your builder or building designer will often have a preferred provider they've worked with before — this can be a practical starting point since the engineer and designer will need to communicate about the results. Alternatively, Engineers Australia's member directory lists registered geotechnical engineers by state.

Ready to Start Your Build?

At Davies, we work through soil test results with clients as part of the early design and costing stage — so you understand what your site classification means for your build budget before anything is committed. If you're in the early stages of planning a new home in northern Tasmania, a feasibility conversation is the right place to start.

Get in touch to talk through your site and project with the Davies team.