Provisional sum: what it is, and what happens when one runs over
The builder's estimate for work that can't be priced yet, and what your state's law says once the actual cost is known.
A blank variation form to print (PDF): NSW · VIC · QLD · WA · SA · TAS · ACT · NT
A provisional sum is the builder's estimate, written into the contract, for work they can't put a definite price on when the contract is signed, even after reasonable inquiries. Victoria (section 3), Queensland (Schedule 1B, section 10) and Tasmania (section 7) define it in their Acts, and in all three the builder warrants that the figure was worked out with reasonable care and skill (Victoria, section 20(2); Queensland, Schedule 1B, section 26(2); Tasmania, section 30(2)). If the work costs more than the sum, the contract price rises by the difference under the contract's provisional sum clause; in Victoria that adjustment isn't a variation under the Act (sections 37(5) and 38(8)).
Not legal advice — your contract's own variation clause matters too.
When a provisional sum runs over: each state's rules
One row per state or territory, across three points: the builder's estimate, what the contract has to show, and what follows once the real cost of the work is known. Cells name the section behind each rule, and say plainly where a state's Act has none.
| State or territory | The builder's estimate | What the contract must show | When the actual cost comes in |
|---|---|---|---|
| New South Wales | The Act says nothing on this | A warning that the price may change, with what changes it, next to the price (section 7(5)) | The Act says nothing on this |
| Victoria | Warranted as worked out with reasonable care and skill on what was known when the contract was made, the site included; a sum below the reasonable cost of the work is an offence, 35 penalty units (sections 20(2) and 21(1)) | A schedule for each sum: the work it covers, a cost breakdown with at least the quantities and the unit cost, and how anything charged above the actual increase is worked out (section 22) | Every invoice or receipt for the work goes to the owner as soon as practicable; the adjustment is outside the variation sections; in a dispute, VCAT can vary a provisional sum (sections 23, 37(5), 38(8) and 53(2)(c)) |
| Queensland | Warranted as worked out with reasonable care and skill on what was known when the contract was made, the site included (Schedule 1B, section 26(2)) | The Act says nothing on this | The Act says nothing on this |
| Western Australia | Stating it below the least the work could reasonably cost is an offence, with a $10,000 penalty (section 12(1)) | The Act says nothing on this | The Act says nothing on this |
| South Australia | The prescribed notice to owners: provisional sums must be fair and reasonable estimates in the circumstances | The notice: "ESTIMATE ONLY" or "THIS PRICE MAY CHANGE" beside a price that may move | The Act says nothing on this |
| Tasmania | Warranted as worked out with reasonable care and skill; a sum resting on the designer's or the owner's written advice, or on a scheduled amount, is taken to meet that (sections 30(2) and 30(3)) | The Act says nothing on this | The Act says nothing on this |
| Australian Capital Territory | The Act says nothing on this | The Act says nothing on this | The Act says nothing on this |
| Northern Territory | The Act says nothing on this | The Act says nothing on this | The Act says nothing on this |
What the Acts say a provisional sum is
Victoria's Act calls it an estimate of the cost of carrying out particular work, the materials for it included, that the builder can't give a definite amount for when the contract is made, after making all reasonable inquiries (section 3). Queensland's definition says the same of contracted services (Schedule 1B, section 10). Tasmania's limits it to work the builder can't price after reasonable enquiries, and lets the builder base it on written advice from the designer or the owner, or on a scheduled amount (section 7).
New South Wales, Western Australia, South Australia, the ACT and the Northern Territory don't define it in their Acts. The prescribed notices that Western Australia and South Australia give owners explain it, and Western Australia's names site works as an example.
Provisional sum or prime cost item
A provisional sum covers work; a prime cost item covers a thing, such as a fixture or fitting not yet chosen. Prime cost item vs provisional sum sets out what each state's law says about both, and the PC sum page works an allowance against the actual cost.
When the actual cost comes in higher
The contract price moves by the difference between the sum and what the work cost, under the contract's own clause on provisional sums. Only Victoria's Act asks the contract for detail on each sum: a schedule with the work it covers, a cost breakdown and how anything charged above the actual increase is worked out, so a margin on an overrun is in the contract from the start (section 22). The builder must also give the owner a copy of each invoice or receipt for the work as soon as practicable (section 23).
Where the estimate was too low from the start, Victoria makes a sum below the reasonable cost of the work an offence (section 21(1)) and lets VCAT vary a provisional sum when it resolves a dispute (section 53(2)(c)); Western Australia makes an understated sum an offence (section 12(1)). In Queensland and Tasmania the builder's warranty of reasonable care and skill covers the estimate (Schedule 1B, section 26(2); section 30(2)), and in Tasmania a sum based on the designer's or the owner's written advice is taken to meet it (section 30(3)).
Putting the adjustment in writing
The form on this site has a box for it: tick "It adjusts a prime cost item or provisional sum", and the document records the adjustment beside who asked for it. On a Victorian job the document's statement says the adjustment is made under the parties' contract, because the Act's variation sections don't apply to it (sections 37(5) and 38(8)). The change to the contract price, the GST and the new contract price are added up as you type, and both parties sign it.
Sources
- Home Building Act 1989 (NSW): section 7(5). Read 07/10/2026.
- Domestic Building Contracts Act 1995 (Vic): sections 3, 20(2), 21(1), 22, 23, 37(5), 38(8), 53(2)(c). Read 07/10/2026, 08/10/2026 and 07/11/2026.
- Queensland Building and Construction Commission Act 1991 (Qld): Schedule 1B, sections 10, 26(2). Read 07/10/2026.
- Home Building Contracts Act 1991 (WA): section 12(1). Read 07/10/2026.
- Residential Building Work Contracts and Dispute Resolution Act 2016 (Tas): sections 7, 30(2), 30(3). Read 07/10/2026.
- Building Work Contractors Regulations 2026 (SA). Read 07/10/2026.
- Home Building Contracts Regulations 1992 (WA). Read 07/10/2026.
Fill in the variation
- $9.90 to send it. Filling it in and the preview PDF are free.
- Both parties sign it. The other party signs from the link we email them, on a phone or a computer — no app, no account.
- Follows your state's rules. It asks for what your state requires and prints its statement, and the PDF ends with a signing record: who signed, when, and from which device.
See a signed example (PDF)A fictional job, signed by both parties.