Provisional Sum Site Conditions: Why They Blow Out
The ground under your block is the one thing nobody priced with certainty. Here is why provisional sums for site works move, and how to plan for the variance instead of being blindsided by it.
Your contract has a number against "site works" or "footings", and it looks firm. It is not. A provisional sum for site conditions is an estimate, not a fixed price, and the ground beneath your block has a habit of disagreeing with it. When the soil report comes back or the excavator hits rock, that estimate gets reconciled against what the work actually costs, and the difference lands on you.
This guide explains what goes wrong below ground, why a provisional sum is uncertain by design, and the questions to ask before you sign. It is written for owners who want to read a contract clearly rather than be surprised by it six weeks into the build.
What a provisional sum actually is
A provisional sum is an allowance your builder puts in the contract for work that cannot be priced precisely when you sign. Site works are the classic example, because nobody knows exactly what is under your block until they dig.
The number is a genuine estimate, but it is still an estimate. When the work is done, the builder reconciles the real cost against the allowance. If the real cost is higher, you pay the difference. If it is lower, you should be credited.
This is different from a PC sum, which covers a supply item you will choose later, like tapware or tiles. A provisional sum covers the labour and works themselves, and site works are where the biggest movements happen.
The Australian Government's plain-English guide to building contracts is a useful starting point for understanding how allowances sit inside the contract sum.
What goes wrong below ground
Site-conditions provisional sums blow out for reasons that are physical, not financial. Here are the common ones.
Soil class
Your block is classified for reactivity, from stable sand through to highly reactive clay. A worse class than assumed means deeper, wider, or more heavily reinforced footings, and the footing system is often the single largest swing in a site-works budget.
Rock
If the excavator hits rock, it has to be ripped or broken out, which is slow and expensive. Rock is rarely confirmed until digging starts, so allowances for it are educated guesses at best.
Fill and slope
A sloping block, or one with uncontrolled fill from a previous owner, can need retaining, extra excavation, or engineered piering. None of this is visible from the street when the allowance is set.
Drainage and water
A high water table, poor stormwater, or a need for subsoil drainage adds cost. Wet weather during earthworks compounds it, turning a clean dig into a churned mess that has to be remediated.
Access and services
A tight block, a long service run to the street, or relocating an existing sewer or power connection all add cost that a generic allowance cannot capture accurately.
None of these is the builder inflating a number to catch you out. They are real, physical unknowns. The problem is that the current system asks you to sign a firm-looking figure for them before anyone has properly investigated the ground.
Why the numbers are uncertain by design
Provisional sums exist precisely because some costs cannot be fixed in advance. A builder who refuses to use them either has to load the whole contract with a fat contingency, which you pay for whether or not it is needed, or carry the risk themselves, which they cannot afford to do on thin margins.
Under the current system, builders finance much of a job out of their own pocket and work on slim margins. They are not in a position to absorb an open-ended hole in the ground, so the risk is passed to you through the allowance.
That makes the provisional sum a shared-risk tool, not a trap. The trouble is visibility. You often see one round number with no breakdown, no soil report behind it, and no clear sense of how far it could move. A realistic site-works allowance can move by tens of thousands of dollars once the ground is known.
The Australian Bureau of Statistics tracks building construction costs, and input costs can be volatile, which widens the gap between an early allowance and the final reconciled figure.
How the blowout reaches your contract sum
When the real cost of site works exceeds the allowance, the increase does not just appear on an invoice. It should flow through as a variation to the contract, with the cost evidenced and recorded against the job.
A variation is the formal change to your contract sum. Treating a provisional-sum overrun as a variation matters, because it forces the cost into the open: what was allowed, what it actually cost, and why.
This is where a clear paper trail protects you. If the soil report, the revised footing design, and the supplier invoices are all attached to the change, you can see exactly what you are paying for. If they are not, you are being asked to trust a number with nothing behind it.
For the broader picture of how changes accumulate during a build, see our guide on variations and cost overruns.
Questions to ask before you sign
You cannot remove ground risk, but you can refuse to sign blind. Ask these before the contract is final.
Is there a current soil report?
Ask for the geotechnical or soil classification report behind the site-works allowance. If the allowance was set without one, treat the figure as a rough placeholder, not a price.
What soil class and conditions are assumed?
Get the assumption in writing: the soil class, whether rock is allowed for, and whether the block is treated as level. Anything worse than assumed becomes a variation.
How is an overrun handled?
Confirm that any increase comes to you as a documented variation with evidence, not a verbal heads-up or a line buried in a later progress claim.
Will I be credited if it comes in under?
A provisional sum runs both ways. If the work costs less than allowed, you should get the difference back. Make sure the contract says so.
How much could it realistically move?
Ask the builder for a sensible range based on the block and the area, not a single figure. A builder who has worked nearby will have a feel for the ground.
If a builder will not put these answers in writing, that tells you something useful before you commit. Our guide on negotiating your building contract covers how to raise these points without souring the relationship.
Planning for variance, not surprise
The healthiest way to think about a site-conditions provisional sum is as a budget with a known wobble, not a fixed cost. Set aside a contingency on top of the contract sum specifically for ground unknowns, and keep it untouched until earthworks are done.
Reading your payment schedule helps here, because site works usually sit in the early stages, before the slab. Knowing when the reconciliation lands lets you prepare the funds rather than scramble for them.
Visibility is the part the current system leaves out. When you cannot see what sits behind an allowance, every adjustment feels like a shock, even when it is legitimate.
On BuildFair, a provisional-sum overrun flows through as a variation against the contract sum, and both builder and owner must approve it under dual approval before it changes what you owe. Variations are the only thing requiring dual approval; ordinary payouts do not. The change, its evidence, and its timing are recorded permanently in a double-entry, hash-chained ledger, so the cause and the paper trail sit in one place. You can read more about how we hold project records on our trust and security page.
This is general information, not legal or financial advice. For a contract dispute, or before you sign anything you are unsure about, speak to a lawyer, your state tribunal such as VCAT or NCAT, or Legal Aid.
FAQ
Frequently asked questions
Is a provisional sum the same as a fixed price?
No. A provisional sum is an estimate for work that cannot be priced precisely when you sign, most commonly site works. It is reconciled against the real cost once the work is done, so the figure can rise or fall. A fixed price does not move. Always check which one your contract uses for footings and excavation.
Why do site-works provisional sums blow out so often?
Because the cost depends on what is under your block, and nobody knows that for certain until they dig. A worse soil class, hidden rock, uncontrolled fill, slope, or a high water table all push the cost up. These are physical unknowns, not the builder inflating the number to catch you out.
Can I be charged more than the provisional sum without my approval?
You should not be charged a higher amount quietly. A genuine overrun should reach you as a documented variation with evidence of the real cost. On BuildFair, a variation requires approval from both the builder and the owner before it changes the contract sum, so you see the change before you owe it.
How much contingency should I keep for site conditions?
There is no single right figure, and this is general information rather than financial advice. As a habit, owners often hold a contingency on top of the contract sum for ground unknowns and leave it untouched until earthworks finish. Ask your builder for a realistic range based on your specific block and area.
What is the difference between a provisional sum and a PC sum?
A provisional sum covers labour and works that cannot be priced precisely yet, like site works. A PC sum covers a supply item you will select later, like tapware or floor tiles. Site-conditions blowouts almost always come from provisional sums, not PC sums.
Will I get money back if site works cost less than allowed?
You should. A provisional sum runs in both directions, so if the real cost comes in under the allowance, the difference should be credited to you. Confirm the contract says this in writing, because not every contract spells out the credit as clearly as it spells out the extra charge.