What Do I Need to Know About My Soil Test?

11th August 2025

Pre-Construction During Construction Post-Construction First Home Buyer Sustainability & Energy Efficiency

In this episode of the Home Building Hub Podcast, Colin Bischof and Darren Brennan break down everything you need to know about your soil test before building. They explain how soil classifications impact your slab design, site costs, and the long-term stability of your new home. A must-listen for anyone preparing to build on a new block of land.

What Do I Need to Know About My Soil Test?

When it comes to building a new home, understanding your soil test is one of the most important steps in the process. It’s the foundation of your foundation — literally. In this episode of the Home Building Hub Podcast, Colin Bischof and Darren Brennan unpack what soil testing is, why it matters, and how it affects your site costs and slab design.

What Is a Soil Test?

Before a builder lays the slab, they must understand what’s beneath your block. A soil test involves a qualified engineer drilling three to five boreholes across your site — typically up to 1.8 metres deep — to analyse the type and condition of the soil.

From there, they extract samples and evaluate:

• Moisture content

• Reactivity and stability

• Soil strength and consistency

• Presence of rock, fill or loose soil

The goal is to determine the right slab design for your home, ensuring stability and long-term durability.

Understanding Soil Classifications

Soil tests are classified using letters that describe the soil’s reactivity and stability. Each class determines the engineering required for your slab and therefore impacts your site costs.

Here’s a quick breakdown:

• Class A – Mostly sand or rock, very stable soil.

• Class S – Slightly reactive clay, still relatively stable.

• Class M – Moderately reactive clay requiring stronger slab design (most common standard).

• Class H – Highly reactive clay, needing additional engineering.

• Class E – Extremely reactive soil requiring significant slab reinforcement.

• Class P – Problematic site with uncontrolled fill, old tree roots or unusual moisture conditions.

Builders often allow for a Class M or H slab in their standard inclusions. However, if your site is classified higher, expect increased site costs to account for additional engineering.

Why Soil Class Matters

Your soil classification directly determines your foundation type and construction cost. For example:

• A-Class sites need simpler, thinner slabs — generally cheaper.

• H or P-Class sites require deeper beams, piers and more steel reinforcement — which increases costs.

Even neighbouring blocks can vary in classification. Factors like previous land use (old dams, fill, or tree roots) can affect your test results and lead to higher costs.

What Affects Your Soil Test Results?

Several environmental and historical factors can influence your soil test and slab design, including:

• Trees and roots – Roots absorb moisture and can cause soil movement near your slab.

• Previous land use – Former dams, filled areas, or demolished structures can alter soil stability.

• Waterlogged or dry soil – Changes in soil moisture can impact its reactivity.

• Uncontrolled fill – If the site was filled without proper compaction, it may require deeper piers.

• Nearby street trees – Common on corner blocks, often overlooked in cost estimates.

Managing Site Costs and Expectations

Site costs can differ significantly between builders — and even between lots in the same estate. That’s because:

• Each engineer’s risk assessment may vary.

• Builders include different slab classifications in their base price.

• The presence of rock, fill or trees can add unexpected costs.

It’s also worth noting that engineers are independent of the builder. Their job is to ensure your slab design meets Australian standards and that your home remains structurally sound.

If your soil test reveals extra work — like deeper piers or a thicker slab — it’s not the builder being difficult; it’s simply the result of the site’s real conditions.

Tips for Home Buyers

• Always ask your builder to explain your soil test report. It includes borehole logs and recommendations.

• Use historical imagery tools (like Nearmap) to see if your site once had trees or dams.

• Understand that site costs are not one-size-fits-all — they’re unique to your block.

• Keep up with proper drainage and landscaping after building to maintain soil stability.

Key Takeaways

• A soil test is essential for determining the right slab design and ensuring a stable foundation.

• Your soil classification (A, S, M, H, E, or P) directly affects your site costs.

• Factors like trees, fill, and previous land use influence soil test outcomes.

• Always review your soil test report and discuss it with your builder to understand potential costs.

• Over-engineering your slab may increase cost but ensures long-term peace of mind.

Glossary of Terms

• Boreholes – Holes drilled into the ground to collect soil samples.

• Reactivity – How much soil expands or contracts with moisture changes.

• Uncontrolled Fill – Soil or debris added to a block without proper compaction or testing.

• Piers – Deep concrete columns supporting the slab to reach stable ground.

• Slab Heave – Movement or cracking of the slab due to soil expansion or contraction.

FAQs

1. Can I change my soil classification? No, soil classifications are based on scientific testing. You can’t request a downgrade to reduce costs.

2. Why is my site cost higher than my neighbour’s? Each block has different soil conditions. Old trees, fill, or previous structures can affect your test results.

3. When does the soil test happen? Usually after your deposit is paid and your land is titled. It’s one of the first steps before final slab design.

4. What happens if my soil test finds rock? If rock is found, additional excavation may be required. The cost will depend on the depth and hardness of the rock.

🎧 Listen or Watch the Full Episode: 👉 What Do I Need to Know About My Soil Test?

About the Home Building Hub Podcast

The Home Building Hub podcast, hosted by industry experts Colin Bischof and Darren Brennan, is Australia’s premier resource for new home buyers. With weekly episodes featuring special guests, the podcast provides objective, high-quality insights into the home building process - completely free and without sales pitches. From understanding financing options to navigating the complexities of building a new home, each episode is packed with actionable advice and tips. Explore more episodes at http://www.homebuildinghub.com.au and join our growing community of informed home buyers.

Disclaimer

This article was generated by AI based on a transcript of our podcast episode and may contain inaccuracies or omissions. Whilst we’re all about providing value to you, this article should not be considered as legal or financial advice. It contains general information only and is based on the content discussed during the podcast episode. This information is relevant to the episode’s release date and may not be applicable at the time of reading. Always seek independent professional advice tailored to your personal situation before making any legal or financial decisions.