Provisional Sums and Prime Cost Items: The Line Items That Break a Fixed Price in Australia

Last updated: 21 May 2026 · By Mossy Tariq, Founder — Property Blueprint Co.

Two homeowners sign what they each believe is a fixed-price renovation contract for the same amount. One finishes within a few hundred dollars of the contract price. The other finishes eleven thousand dollars over it. Neither builder did anything wrong, neither contract was breached, and the difference between the two outcomes was decided entirely by a handful of line items most homeowners never read closely: the provisional sums and prime cost items.

These are the parts of a renovation quote that are not actually quoted. They are estimates the builder has inserted as placeholders — an allowance for work or materials that could not be priced precisely when the contract was drawn up. The contract looks fixed. Those line items are not. And because they are buried inside a document that presents itself as a single agreed price, the homeowner who does not know to interrogate them signs a number that was never the real number. It is the same gap that runs through every renovation: the difference between the prepared homeowner and the unprepared one is whether they read the soft numbers before signing or discover them after. Knowing how to read for them is part of the broader skill of reading a renovation quote — but provisional sums are deep enough, and expensive enough, to be worth understanding on their own.

This article is about what to do with them: how to tell a reasonable allowance from a low-balled one, what the contract actually permits a builder to charge when an allowance is exceeded, and how to firm up the soft numbers before you sign rather than after the invoice arrives.

A provisional sum isn't a price you've been quoted.
It's a price you've agreed to find out later.

What follows is the practical distinction between the two item types, why they are the structural weak point of a fixed-price contract, the builder's-margin mechanic that turns an overrun into more than the overrun, the questions that firm up an allowance before signing, and the rule that decides when a change becomes a variation you have the right to refuse.

What is the difference between a provisional sum and a prime cost item?

A prime cost item, or PC item, is an allowance for the supply of a specific product the homeowner has not yet chosen — tapware, a cooktop, floor tiles, a vanity. The builder knows the product is needed and roughly what it will cost, but cannot price it exactly because the homeowner has not selected it yet. The allowance is a placeholder for the supply cost of that not-yet-chosen item.

A provisional sum, or PS, is broader. It is an allowance for work whose full scope cannot be priced when the contract is signed — excavation, rock removal, the labour for tiling, repairs behind a wall that has not been opened. The Housing Industry Association draws the distinction cleanly: a PC item covers the supply of a product, while a provisional sum covers work the builder genuinely cannot give a definite price for at signing. The practical difference for the homeowner is that a PC item is mostly about choice — what you pick — and a provisional sum is about discovery — what the site turns out to require.

The reason the distinction matters is that the two behave differently when the real cost lands, and a homeowner who treats them as the same thing misreads both. A PC item moves with your selections. A provisional sum moves with what the work uncovers. Both are honest contractual devices when used properly. Both are also where the unprepared homeowner's budget quietly comes apart.

Why provisional sums are where a "fixed-price" quote stops being fixed

A fixed-price contract is not fixed in the places it carries provisional sums and prime cost items. It is fixed everywhere else and estimated there. This is not a flaw in the contract — it is how a builder honestly handles work that genuinely cannot be priced at signing, and the standard residential contracts published by bodies like Master Builders Australia build these allowances in as a recognised device — but it is routinely misunderstood as a guarantee it was never meant to be.

The risk is the soft number disguised as a hard one. A homeowner reads a contract with a headline price, sees the word "fixed", and assumes the figure at the bottom is the figure they will pay. Inside that figure sit several allowances that are estimates — and an allowance that is too low makes the headline price look more competitive than the project actually is. A builder competing for the job has a quiet incentive to keep those allowances lean, because the contract total is what the homeowner compares against rival quotes, and the overruns do not appear until the work is underway and the homeowner is committed.

This is why the allowance has to be honest. Industry guidance is explicit that a provisional sum should be a reasonable estimate of what the builder believes the work will cost, based on the information available at signing — not a placeholder number chosen to win the contract. The distinction between a fixed-price contract and the variations that adjust it is set out in the consumer guidance that state building regulators publish for homeowners before they sign. A homeowner reading three quotes where one carries conspicuously lower allowances than the other two is not necessarily looking at the cheaper builder. They may be looking at the builder whose final price will land closest to the others once the soft numbers firm up. Reading the allowances, not just the headline total, is what tells the difference — the same discipline that turns a quote from a price into a negotiating position.

What happens when a provisional sum is exceeded — and the margin no one mentions

When the real cost of a provisional sum or prime cost item comes in higher than the allowance, the homeowner pays the difference. That part most people expect. What they do not expect is the second charge sitting on top of it: the builder's margin, applied to the difference.

Under a standard residential building contract, if the actual price exceeds the allowance, the contract price is adjusted by the overrun plus the builder's margin on that overrun — a percentage set in the contract, and where none is stated, commonly 20 percent. So an allowance that comes in two thousand dollars over does not cost the homeowner two thousand dollars. At a 20 percent margin it costs two thousand four hundred. The margin compounds the very item that was already underestimated, and it does so on the part the homeowner had no way to control.

The mechanic runs the other way too, and this is the part homeowners forget to enforce. If the actual price comes in below the allowance, the difference is credited back to the homeowner in the next progress claim. A provisional sum is a two-way adjustment, not a one-way ceiling. The builder who is quick to add the overrun and slow to credit the underrun is not following the contract — they are following the half of it that suits them. A homeowner who knows the allowance cuts both ways is a homeowner who checks the progress claim for the credit, not just the charge.

How to interrogate a provisional sum before you sign

Every provisional sum and prime cost item is negotiable before signing and locked after it. The work is done in the days before the contract is signed, while the allowances are still soft and the builder still wants the job. Five questions firm them up.

1. Ask for every allowance to be listed by name with its own dollar figure. A contract with a single round-figure provisional sum and no breakdown is a contract you cannot interrogate — insist each item is named individually with its own allowance, because an allowance you cannot see is one you cannot question.
2. Ask what each allowance is based on. Request the basis of the estimate — a supplier quote, a measured area, a comparable recent job — because a number with a basis is a reasonable estimate and a number with no basis is a guess the homeowner inherits the risk of.
3. Ask which allowances can be converted to fixed prices now. Many provisional sums exist only because a selection has not been made, so make the selection — choose the tapware, the tiles, the cooktop — and ask the builder to convert that PC item from an allowance to a fixed line, removing the soft number entirely.
4. Ask what the builder's margin on overruns is. Confirm the margin percentage that applies when an allowance is exceeded and get it stated in the contract, because the difference between a stated margin and the default can be several percent on every overrun across the job.
5. Ask for the process and notice you will receive before an overrun is charged. Establish in writing that you will be notified, with an itemised cost, before any allowance is exceeded and charged — so an overrun arrives as a conversation before the work, not as a line on an invoice after it.

An allowance that survives those five questions is one you can trust. An allowance the builder is unwilling to itemise, justify, or convert is telling you where the contract's real risk is sitting — and it is sitting with you.

When a prime cost item becomes a variation — and when it doesn't

This is the distinction that decides whether the homeowner has the right to refuse a charge, and it is the one builders most often get wrong — sometimes innocently, sometimes not. A prime cost or provisional sum coming in over its allowance is not a variation. It is an adjustment, handled through the contract's progress-claim mechanism, and it does not require the homeowner's signature to take effect, because the work was always in scope and only the price moved.

A change in scope is different. If the homeowner changes what the item is — swapping an electric cooktop for a gas one that now requires gas lines and certification, or upgrading from the specified tile to a different product entirely — that is a variation, not a PC adjustment. Industry contract guidance is clear that a change of specification is a variation, and a variation must be put in writing and agreed before the work proceeds. That requirement is the homeowner's protection: a true variation gives you the right to see the price and approve it first.

The trap is the builder who reframes an ordinary overrun as a variation to add charges, or reframes a genuine scope change as a simple adjustment to avoid getting a signature. Knowing which is which is what lets a homeowner hold the line — refusing to sign a variation that should have been an in-scope adjustment, and refusing to accept an "adjustment" that is actually an unapproved scope change. The discipline of holding the contract and payment hold points is exactly where this distinction is enforced.

Where provisional sums sit in the bigger picture

Provisional sums and prime cost items are not a quoting trick to be feared. They are a legitimate way to handle the parts of a renovation that genuinely cannot be priced at signing — and a fixed-price contract without any of them, on a project with real unknowns, is often a contract that has hidden its contingencies somewhere less visible. The goal is not to eliminate them. The goal is to make the soft numbers as small and as scrutinised as possible before the contract locks.

This sits inside the contract phase of the renovation — the highest-leverage hold point in the whole project, because every later cost inherits whatever the contract locked in. Provisional sums are simply the most technical part of that phase, and the part where a few hours of scrutiny before signing saves the most money after. It is one decision point inside The 12-Phase System, Property Blueprint Co.'s named mechanism for running a renovation from the first quote conversation to final sign-off — and it applies to every room, indoor and outdoor, since an outdoor renovation often carries the largest provisional sums of all in its excavation and site-preparation allowances.

If the cost baseline is the right first step, use the free Renovation Cost Calculator — a trade-by-trade estimate for the specific renovation, in under 5 minutes, before any trade has quoted.

Frequently asked questions

What is the difference between a provisional sum and a prime cost item?

A prime cost item is an allowance for the supply of a specific product the homeowner has not yet selected — tapware, a cooktop, tiles, a vanity — where the builder knows the product is needed but cannot price it until it is chosen. A provisional sum is an allowance for work whose full scope cannot be priced at signing, such as excavation, rock removal, or repairs behind an unopened wall. A prime cost item moves with the homeowner's selections; a provisional sum moves with what the work uncovers.

Does a provisional sum make a contract not fixed-price?

A fixed-price contract is fixed everywhere except in its provisional sums and prime cost items, which are estimates rather than firm prices. This is not a flaw — it is how a builder honestly handles work that genuinely cannot be priced at signing — but it means the headline contract total is not a guarantee. The soft numbers can move, so the homeowner who treats a fixed price with large allowances as a fixed total has misread the contract.

What happens if a provisional sum is exceeded?

If the actual price exceeds the allowance, the contract price is adjusted by the overrun plus the builder's margin applied to that overrun — a percentage set in the contract, commonly 20 percent where none is stated. So an allowance that runs over by two thousand dollars can cost two thousand four hundred. The adjustment also works in reverse: if the actual price is below the allowance, the difference is credited back to the homeowner in the next progress claim. A provisional sum is a two-way adjustment, not a one-way ceiling.

Is a provisional sum overrun a variation?

No. A provisional sum or prime cost item coming in over its allowance is an adjustment handled through the progress-claim mechanism, not a variation, because the work was always in scope and only the price moved. It does not require the homeowner's signature to take effect. A change in scope — swapping an electric cooktop for a gas one, or upgrading to a different specified product — is a variation, which must be put in writing and agreed before the work proceeds.

How do I stop provisional sums blowing my budget?

Firm up the allowances before signing, while they are still negotiable. Ask for every allowance to be listed by name with its own dollar figure, ask what each estimate is based on, convert as many prime cost items as possible to fixed prices by making the selections now, confirm the builder's margin on overruns in writing, and require written notice with an itemised cost before any allowance is exceeded and charged. An allowance that survives that scrutiny is one you can trust.

Should I be suspicious of a quote with low provisional sums?

A quote carrying conspicuously lower allowances than competing quotes is not necessarily the cheaper builder — it may be the builder whose final price lands closest to the others once the soft numbers firm up. A provisional sum should be a reasonable estimate based on the information available at signing, not a placeholder chosen to win the job. Comparing the allowances line by line, rather than just the headline totals, reveals which quote is genuinely competitive and which is competitive only on paper.