PC Sum and Provisional Sum: The Allowances in Your Contract

The two terms get used loosely, but they cover different kinds of uncertainty. Knowing which is which tells you how much risk sits with you.

A useful rule of thumb: PC sums are about products you will choose, provisional sums are about work whose extent is still unknown.

A quote with thin allowances looks cheaper than a quote with realistic ones, even when the finished house costs exactly the same. If two builders price the same plans and one allows $40 a square metre for tiles while the other allows $80, the first quote looks better on the headline number. You only feel the difference at selection time, when the tiles you actually want cost what they always cost.

This is not always a trick. Builders are squeezed inside a system that rewards the lowest headline price, so a realistic allowance can lose them the job to whoever lowballed it. The current system pushes everyone toward optimistic numbers and sorts out the truth later, which is exactly the pattern that turns a comfortable budget into a stressful one.

Provisional sums for site works are a particular trap because the unknowns are genuinely unknown until excavation. A low site-works allowance is easy to write and hard to challenge before anyone has dug. If the ground turns out to be reactive clay or full of rock, the true cost can dwarf the allowance, and that difference is yours to fund. We cover this specific risk in site conditions and provisional sums.

Australia's own data shows how much input prices move over a build. The Australian Bureau of Statistics tracks producer prices for construction materials, and the cost of timber, steel, and fittings can shift noticeably across a single project. You can see the trend on the ABS producer price indexes. An allowance set at the start of a long build is a guess about a moving target.

You cannot eliminate allowances, but you can make them honest before you commit. Work through each one with your builder and write the answers into the contract.

None of this is about distrust. It is about converting guesses into decisions while you still have leverage, which is before you sign.

Allowances are reconciled as the build progresses, not all at once at the end. When you make a selection or the site work is measured, the difference between the allowance and the real cost is adjusted into the contract sum. That adjustment is a change to the price, so it should be documented like any other change, with the figures shown and your agreement recorded before it is locked in.

This matters because an adjustment to a PC sum or provisional sum is functionally a variation to the contract sum. Treat it with the same scrutiny: see the actual cost, see the difference against the allowance, and approve it in writing. Vague verbal updates about being a bit over on tiles are how budgets drift without anyone deciding to let them.

Keep your own running tally. Every time an allowance is trued up, note the original figure and the new one, and watch the total. A series of small overruns across many allowances can add up to a number large enough to matter, and you want to see it building rather than discover it at handover. Understanding how the staged payment schedule works alongside these adjustments helps you keep the whole picture in view.

BuildFair does not set your allowances or tell you whether a number is fair; that is between you, your builder, and your own adviser. What the platform does is record the spec and the variations that allowances feed into, so the price story is visible rather than scattered across emails.

The agreed scope lives in the spec sheet and materials specification that form part of the tender both parties sign in-app. When an allowance is trued up, that adjustment runs through the same variation flow as any other change, and variations require dual approval: the builder and the owner both have to agree before the contract sum moves. Approved changes are written to a double-entry, hash-chained ledger, so every adjustment to the price has a permanent, tamper-evident record you can both point back to.

Meanwhile the money side stays orderly. Owner deposits and progress payments are held in regulated custody with BuildFair banking partner Kobble, separate from the builder operating account, and released only on verified release conditions. Kobble operates under AFSL 545391 (Yondr Money Pty Ltd). That separation means an allowance adjustment is a recorded decision about the price, not a quiet movement of your money. You can read more on the how BuildFair protects funds page.

PC sums and provisional sums are the honest unknowns in a building contract, and they are also the easiest place for a quote to look cheaper than the finished house will be. The fix is not to fear them; it is to pressure-test them before you sign and to document every adjustment while the build runs.

Walk each allowance back to a real product or a real soil report, count how much of your price is sitting in placeholders, and lock down every selection you can in advance. Then treat each true-up as a decision you approve, not a number that happens to you.

This guide is general information about how PC sums and provisional sums work in Australian residential building. It is not legal, financial, or tax advice. For advice on your specific contract or a dispute over an allowance adjustment, speak to a building lawyer, your state tribunal such as VCAT or NCAT, or Legal Aid. For general guidance on contracts and consumer rights, business.gov.au is a useful starting point.