Two blocks in the same street can look identical and cost tens of thousands of dollars apart to build on. The difference is under the grass.
A soil test tells the engineer how much the ground on your block moves as it wets and dries, and the engineer designs your concrete slab to suit. The result is your site’s soil classification. In Dubbo, the most reactive class, an E site, can add anywhere from about $40,000 to $90,000 to the cost of a build.
For a first home buyer, that’s often the difference between a budget that works and one that doesn’t. This guide draws on Michael Fuller, licensed builder at Robinson & Fuller Homes. It explains what the soil classes mean, when you’ll find out yours, and how to get a read on the soil before you buy.
The short version
- Every block gets a site class from A (stable) to E (extremely reactive), or P for problem sites.
- The more the soil moves, the stronger and more expensive the slab.
- An E site in Dubbo can add about $40,000 to $90,000 to a build.
- Ask the developer or agent about soil tests before you buy the block.
Jump to:
- What each soil class means
- How a soil test works
- What an E site costs
- Checking the soil before you buy
What is soil classification?
Your soil classification tells you how much the ground on your block is expected to move, using the site classes in Australian Standard AS 2870. That class decides how strong your slab and footings need to be.
The site classes at a glance
| Site class | What it means | What it does to your slab |
|---|---|---|
| A | Mostly sand or rock, with little or no ground movement | The simplest slab design |
| S | Slightly reactive clay, with slight movement as the soil wets and dries | A standard slab |
| M | Moderately reactive clay or silt | A stiffer slab than an S site |
| H1 | Highly reactive clay, with high movement | Deeper beams and more reinforcement |
| H2 | Highly reactive clay, with very high movement | Deeper beams and more reinforcement again |
| E | Extremely reactive clay, with extreme movement | A heavily engineered slab, and the biggest cost increase |
| P | A problem site, such as soft soil, fill, trees or erosion | A design specific to that site from a structural engineer |
The descriptions follow the AS 2870 classes as explained by Monash City Council’s guide to soil investigation.
Why ground movement matters for a slab
Uneven ground movement can bend and crack a slab, so your slab has to be stiff enough to ride it out. Clay soil swells when it gets wet and shrinks as it dries out, so a slab sitting on clay rises and falls with it. The trouble starts when one part moves more than another.
Reactive soil in plain terms
Reactive soil is clay that changes volume a lot with moisture. The more reactive the clay, the further the ground moves between a wet winter and a dry summer, and the higher the site class it gets.
What is a soil test and who orders it?
A soil test for building takes samples from your block to work out its site class. Robinson & Fuller Homes arranges the soil testing on your block as part of the site investigations before your slab is designed.
What happens during a soil test
A geotechnical tester drills a small number of bore holes across the block, records the soil layers and assesses how reactive the clay is. The findings go into a site classification report, which gives the block a single class from A to E, or P.
How the engineer uses the result
The engineer takes the site class and designs a slab and footings strong enough for it. On a more reactive site, that usually means deeper and wider edge beams, more steel reinforcement and more concrete. On some sites it can also mean piers down to firmer ground.
How much extra does an E-class site cost to build on in Dubbo?
Michael says an E site in Dubbo can cost anywhere from about $40,000 to $90,000 extra, compared with a site that doesn’t need the heavier slab. Where you land in that range depends on how reactive the clay is and the design of the house.
What that money pays for
The money goes into a heavier slab and the engineering behind it. None of it shows once the house is finished, which is part of why it surprises people.
Why E sites are common around Dubbo
Michael says there are lots of places around Dubbo where his team has to build on E sites. Robinson & Fuller Homes has built quite a few E-site slabs across Dubbo, so they know how to plan and price one.
What an E-site slab does to your $750,000 grant cap
The $10,000 First Home Owner Grant only applies when your land, building contract and variations total $750,000 or less. A slab that heavy can push a build that looked comfortable over that line. Our guide to the First Home Owners Grant in NSW explains how the cap is counted.
Wondering what the soil is like on the block you’re looking at? Ask us what we know about the area before you sign.
When do most people find out their soil class?
Most first home buyers find out when the soil test comes back, which can be months into planning their build. By then they’ve often bought the land and settled on a house design.
Months into planning, when the test comes back
Michael has had clients spend months planning a build with him, only for the soil test to come back a more extreme class than expected. A building and pest inspection is standard when you buy an established home, but nothing equivalent tells you about the soil on a vacant block unless you ask.
Why some buyers have had to walk away
Michael says some clients have had to walk away from their build because they couldn’t afford the stronger slab. Finding out about the soil early is the simplest way to avoid that.
How can you find out about the soil before you buy?
Ask the developer or the real estate agent whether soil tests have been done on that block or nearby lots, before you sign the land contract. A known soil class lets your builder price the slab with far fewer surprises.
Ask the developer or agent
Developers of new estates often hold geotechnical reports for the subdivision, so ask for them by name.
Ask your builder what they know about the area
A builder who works around Dubbo will often know which areas tend to be more reactive. Michael says his team can sometimes point clients towards spots with better soil. Our guide to buying land to build a house in Dubbo covers the other checks to make on a block.
Leave a buffer until the test is back
Until your soil class is confirmed, keep some room in the budget for a heavier slab. If the result comes back better than feared, that money goes back into the house.
Frequently asked questions
What does H2 soil classification mean?
H2 is highly reactive clay with very high ground movement, one step below E. A slab on an H2 site needs deeper beams and more reinforcement than one on an H1 site.
Can you build on a class P site?
Usually, yes. A class P site doesn’t fit the standard classes, so a structural engineer designs a footing system for that particular site. The cost depends on what’s made the site a problem, such as fill, trees or soft ground.
Does a reactive site need looking after once I move in?
Yes. Reactive clay moves most when the moisture around the slab changes unevenly. Keeping drainage clear, fixing plumbing leaks quickly and not watering garden beds right up against the slab all help keep the ground steady.
Is an E site a reason not to buy a block?
Not on its own. The question is whether the extra slab cost still leaves the land and house inside your budget and your grant cap.
Ask About the Soil Before You Sign
If you’re looking at a block around Dubbo, Robinson & Fuller Homes can tell you what we know about the area and help you plan for the slab before you commit.
Call (02) 6882 7444, request a free quote, or see how we build in our process.
Written by the team at Robinson & Fuller Homes.
Disclaimer: This article is general information only. Your slab design depends on the site classification report and engineering for your block.