Soil Classification and Site Costs
Understanding what lies beneath the surface is vital for budgeting and structural integrity.
Your deposit is only part of it. Duty, legal fees and moving costs sit alongside it.
The Importance of a Soil Report
Before a single brick is laid, a geotechnical engineer must perform a soil test on your block. This process involves drilling boreholes to collect samples from various depths. The purpose is to determine how the soil reacts to moisture changes, which is known as its reactivity. In Victoria, soil is classified under Australian Standard AS 2870. The classification dictates the type of foundation or slab required to support your home safely. If the soil is highly reactive, it will expand when wet and contract when dry, potentially causing the house to crack or move if the slab is not designed correctly.
A soil report also identifies the presence of rock, fill, or groundwater. Finding rock beneath the surface is a common cause of budget blowouts in Victoria because it requires specialised heavy machinery to excavate for footings or underground services. Conversely, if the site contains 'uncontrolled fill' from previous building works, the footings may need to be deeper to reach natural, stable ground. These variations are rarely covered in a base price from a builder and are instead listed as provisional sums or variations once the final soil test results are available to the engineering team.
Understanding Site Costs in Your Quote
Site costs are the expenses associated with preparing your land for construction. These include clearing vegetation, levelling the ground, and connecting services to the house. Many builders offer a 'fixed site cost' guarantee to provide certainty. However, it is essential to read the fine print to understand what is included. A fixed price usually covers standard items based on a preliminary soil report but may exclude unexpected findings like large boulders or buried debris. If your site costs are not fixed, the builder will use provisional sums, which are estimates that can increase if the actual work is more difficult.
Provisional sums for site costs represent a significant risk for buyers. If the builder hits rock and the contract allowed 5,000 dollars for excavation, but the actual cost is 15,000 dollars, you must pay the difference. To mitigate this risk, some buyers choose to pay for their own independent soil and contour survey before signing a contract. This allows you to provide the results to multiple builders for more accurate quoting. Always ensure that the site costs listed in your contract include items like temporary fencing, sediment control, and the removal of excess soil from the site after excavation.
Common Factors That Increase Site Costs
Several factors can lead to unexpected expenses during the site preparation phase. One major factor is the need for a 'bored pier' foundation if the soil is particularly unstable or if there are large trees nearby. Trees can suck moisture out of the ground, causing the soil to shrink unevenly. To counter this, engineers often specify deep concrete piers to anchor the slab to more stable soil layers. Another common expense is a 'crane requirement' if the site has limited access. If the builder cannot get a standard concrete pump or delivery truck onto the block, you will face additional hire fees for specialised equipment.
Environmental requirements can also impact your budget. If your block is in a Bushfire Management Overlay, you may need to pay for upgraded windows, ember protection, and specific building materials to meet the required Bushfire Attack Level. Similarly, if your land is in a flood prone area, the floor level of the house may need to be raised significantly above the natural ground level. These regulatory requirements are non negotiable and must be factored into your initial financial planning. Always check the planning property report on the VicPlan website to identify these overlays early in the process.
- Geotechnical soil classification
- Presence of rock or uncontrolled fill
- Need for bored piers or deepened footings
- Removal of excess spoil from the site
- Bushfire Attack Level (BAL) requirements
Contract Tip
Ask your builder to provide a 'fixed price' for site costs rather than a 'provisional sum' to avoid surprise invoices during the early stages of construction.
The Rock Surprise
| Class | Type | Reactivity |
|---|---|---|
| A | Sand or Rock | Little to no movement |
| S | Slightly Reactive | Slight movement |
| M | Moderately Reactive | Moderate movement |
| H1/H2 | Highly Reactive | High movement |
| P | Problem | Subject to abnormal moisture |
Quick knowledge check
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