Provisional Sums and Prime Cost Sums Explained (AU)
A provisional sum is an allowance in a building quote or work contract you cannot fully price at the time, such as excavation in unknown ground. It lets you give the client a complete price now and adjust that one line later when the real cost is known. Used well, provisional sums and prime cost sums are how you quote confidently before every decision is made. Used badly, they are where Australian residential jobs turn into arguments.
This guide explains what each one is, how they differ, and how to show them on a quote so the client is never surprised. If you price cost-plus work, they matter even more, because the whole job runs on allowances.
What is a provisional sum?
A provisional sum covers work that is in the scope but cannot be fully priced yet. You know the work has to happen; you just cannot pin the cost until you get on site or get more information. Common examples are excavation where the ground is unknown, service connections, or repairs to something you cannot see until you open it up.
You put a realistic figure against the line, label it clearly as a provisional sum, and adjust it to the actual cost when the work is done. The client pays the real amount, not the guess.
What is a prime cost sum?
A prime cost sum, often shortened to PC sum, is an allowance for an item the client has not chosen yet. The labour to install it is priced, but the item itself is a placeholder. Think tapware, tiles, appliances, door hardware or light fittings. You set an allowance based on a sensible product, and when the client makes their selection, you adjust the line up or down to the real price.
Provisional sum vs prime cost sum
The two get mixed up, but the difference is simple.
- A provisional sum is about unknown work. You do not yet know how much the work will cost.
- A prime cost sum is about an unchosen item. You know the work, but the client has not picked the product.
A quick example. On a bathroom renovation, the cost to dig out and replace a mystery drain is a provisional sum, because you do not know what is under the floor. The allowance for the vanity the client has not chosen is a prime cost sum, because the item is still up in the air.
How to show them on a quote
Getting the presentation right is most of the job. Do this and you avoid the classic end-of-job dispute.
- Give provisional and prime cost sums their own section, separate from your fixed-price lines.
- Label each one clearly so nobody mistakes an allowance for a fixed price.
- State what the allowance is based on, for example “PC sum for tapware based on a mid-range mixer”.
- Explain in one sentence that the final cost is adjusted to the actual price or selection.
- Show GST correctly across allowances the same way you do on fixed lines.
The most common mistake is setting an allowance too low to make the total look sharp. It wins the job today and burns the client at selection time, when the tiles they actually want cost double the allowance. Keep every allowance honest.
Why this matters for cost-plus jobs
On a fixed-price job, allowances are a handful of lines. On a cost-plus building contract, the whole job runs on actual cost plus margin, so clear allowances and a clean record of what each line was based on are the difference between a smooth claim and a fight. If you build homes, the same discipline runs through every quote, which is why residential construction estimating software treats allowances as their own type rather than as ordinary lines.
How QR-rise handles allowances
QR-rise treats provisional sums and prime cost sums as their own line types, clearly separated on the quote, so the client can see at a glance what is fixed and what is an allowance. When a selection is made, or a cost is confirmed, you adjust the one line and the totals recompute server-side, so the number the client sees always matches the detail behind it. That is how you quote before every decision is locked in without losing the thread later. To see where allowances sit in a full quote, start with our construction quote template, or learn the whole pricing flow in how to quote a house build.
Build one real estimate with labelled allowances and see how much cleaner your next selection conversation is. Start your 30-day Pro trial, or begin on the free capped plan.
Frequently asked questions
What is a provisional sum in building?
A provisional sum is an allowance in a quote or contract for work that is in scope but cannot be fully priced yet, such as excavation in unknown ground. You put a realistic figure against it and adjust to the actual cost once the work is done.
What is the difference between a provisional sum and a PC sum?
A provisional sum covers unknown work, where you do not yet know the cost. A prime cost sum covers an item the client has not chosen yet, where the labour is priced but the product is a placeholder. Both are adjusted to the real figure later.
Do you charge GST on a provisional sum?
Yes. Provisional and prime cost sums are shown with GST the same way as your fixed-price lines, so the GST-inclusive total reflects the whole job.
