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Choose a fixed-price (lump-sum) contract when your plans, selections and site information are sufficiently complete and you need a more predictable committed price. Consider cost-plus only when the work cannot be priced reliably at signing and you can actively check invoices, records and budget changes. Neither structure guarantees that the amount you first see is the amount you finally pay. A fixed price can change through variations, allowances and contract-permitted events; cost-plus has no guaranteed final total by definition. Your state or territory, contract date, project type, value and special conditions can change what is permitted and how payments must work.
What the two contract structures actually mean
Fixed-price or lump-sum
Under a fixed-price contract, the builder agrees to carry out the defined work for an agreed price. The price is predictable only in relation to the scope and assumptions written into the contract. If drawings, specifications, inclusions, exclusions and site information are complete, the lump sum gives you a clearer commitment than an open-ended arrangement.
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“Fixed” does not mean an unconditional cap. The final account can move when you select an item above its allowance, when a provisional sum proves inadequate, when you request a change, or when the contract allows an adjustment for a defined event. A quote with a higher lump sum may include more realistic allowances or contingency than a cheaper-looking quote. Compare what each price buys and which party carries each risk, not just the headline number.
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Cost-plus
With cost-plus, you pay eligible project costs plus the builder’s fee or margin set out in the contract. Eligible costs can include direct construction expenses and specified indirect costs, but the contract must define them precisely. The builder’s fee might be a percentage, a fixed amount or another agreed calculation. The final cost is not guaranteed; Building Commission NSW describes it as a contract used where the project’s nature prevents the final cost from being calculated.
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Because the total depends on actual spending, you need a transparent calculation method, an estimate where required or recommended, records supporting each claim, regular budget reporting and a process for approving decisions that increase cost. NSW guidance says cost-plus progress claims should be supported by receipts or other verifying documents for relevant contracts.
Which structure is more likely to suit your project?
Choose fixed-price when the scope is genuinely ready to price
A lump sum is usually the better fit when architectural and engineering information is resolved, finishes have been selected, access and logistics are known, and the site has been investigated enough for the builder to make credible assumptions. It also suits owners who value a committed amount and do not want to audit every supplier invoice.
- Plans and specifications describe the work in enough detail for competing builders to price the same job.
- Kitchen, bathroom, lighting, flooring, appliances, doors, windows and other significant selections are made or covered by realistic allowances.
- Site access, demolition, services, ground conditions and approvals have been investigated.
- The contract lists inclusions, exclusions, prime-cost items and provisional sums rather than leaving them in informal emails.
- You can follow the variation procedure and make decisions quickly when a change is unavoidable.
A fixed price can include a builder’s contingency for known risks. That can make it look higher than a cost-plus estimate even though the risk allocation is more favourable to you. Reconcile scope and allowances before deciding that one quote is cheaper.
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Cost-plus may fit investigative, remedial or otherwise uncertain work where the extent of demolition, hidden damage, ground conditions or design development cannot reasonably be established before work starts. It can let the project proceed while facts are uncovered, but it transfers more cost risk to you.
You should have enough time and capability to review invoices, payroll or subcontractor evidence where relevant, approve changes and monitor the forecast. Before signing, agree the fee or margin, eligible costs, record format, audit or inspection rights, payment frequency, budget updates and any cost-control mechanism. If you cannot perform that oversight, an open-ended structure is unlikely to provide workable control.
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Compare the contracts on risk, not labels
| Decision point | Fixed-price/lump-sum | Cost-plus |
|---|---|---|
| Price at signing | An agreed price for the defined work | Actual eligible costs plus the agreed fee or margin |
| Final-total certainty | More predictable, but subject to variations, allowances and contract-permitted adjustments | No guaranteed final total |
| Best starting condition | Plans, selections, site information and scope are sufficiently complete | The project cannot yet be priced reliably |
| Owner’s main task | Check scope, exclusions, allowances and variation terms | Check invoices, records, approvals, forecasts and fee calculations |
| Main exposure | Under-scoped work, low allowances and extra-cost variations | Uncontrolled actual costs and a fee that grows with the cost base |
| Progress claims | Check claimed work against the contract and applicable rules | Check work or cost evidence and the contract’s claim process |
Use the same questions for every quote: What work is included? What is excluded? Which amounts are allowances? Who pays if an allowance is exceeded? What happens if it is underspent? What margin is added? When must a variation be approved? What evidence accompanies a claim? Which events change time as well as price? A comparison that does not answer those questions is not a like-for-like comparison.
Why a “fixed” price can still rise
Prime-cost items
A prime-cost item is an allowance for an item whose final selection or purchase cost is not settled when the contract is signed. Sanitaryware, appliances, tiles, tapware and light fittings are common examples. If you choose an item that costs more than the allowance, the difference is added under the contract’s pricing rules. Ask whether the builder’s margin is added to the difference and how a credit is calculated when the actual item costs less.
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A provisional sum is an estimate for work that has not been fully defined. Excavation, rock removal, drainage or repair work can fall into this category. A low provisional sum reduces the initial price but can produce a substantial adjustment once the work is known. Victoria’s Building and Plumbing Commission warns that provisional sums can lead to unexpected costs.
For every provisional sum, ask what assumptions produced the allowance, what evidence will establish the actual cost, whether labour and materials are both covered, and what margin applies. Require the contract to explain how the amount is adjusted and who approves expenditure above it.
Variations and owner decisions
A variation is a change to the agreed work, materials, method, price or time. It may be owner-requested, required by an authority, or triggered by a site condition covered by the contract. Do not rely on a verbal agreement. NSW guidance says variations must be in writing and signed by both parties; Victorian guidance also stresses documenting the details and cost of changes. Apply the law and contract governing your project.
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A useful variation document identifies the original requirement, the proposed change, itemised cost, builder margin, effect on completion date, revised allowance or credit, and signatures or electronic approvals. Ask for that document before the changed work starts unless an emergency or the contract’s urgent-work clause makes that impossible.
Cost-plus controls that should be written into the contract
Define eligible costs
List the costs the builder can recover: subcontractor invoices, materials, equipment hire, permits, waste, insurance or specified site overheads, for example. State whether builder employees, project management, travel, financing, rework and defects are included or excluded. A vague phrase such as “all costs” does not tell you what you will pay.
Set the fee or margin calculation
Specify whether the fee is a percentage of eligible costs, a fixed amount, a tiered amount or another formula. Clarify whether the percentage applies to subcontractor variations, allowances, taxes, discounts, credits and the builder’s own labour. Require the calculation to appear on each claim so you can reproduce it.
Require evidence and reporting
Agree the records supplied with each claim: invoices, receipts, timesheets, purchase orders, subcontractor statements and proof of payment where appropriate. Set a regular reporting date and require a forecast of cost to complete, committed costs, approved variations, contingency use and expected completion date. NSW guidance specifically refers to receipts or other verifying documents for cost-plus progress claims.
Install approval gates
Set a dollar threshold above which the builder must obtain written approval, and identify who may approve on your behalf. Require competitive quotes for significant procurements where practical, while allowing urgent work needed to protect people or property. Decide how disputed costs are held while the undisputed portion is paid.
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Australian rules differ by jurisdiction
There is no single national threshold or permission rule that can safely be applied to every residential project. Confirm the state or territory, contract date, project type and contract value before relying on a rule. A regulator’s current consumer guide is a starting point, not a substitute for contract-specific advice.
Victoria
Consumer Affairs Victoria states that the prohibition on cost-plus methods applies below $1 million for domestic building contracts entered into on or after 1 August 2017, with limited circumstances for investigative work when renovating an existing house. The threshold and exceptions are Victorian requirements; they are not an Australia-wide rule. The Victorian consumer checklist advises seeking legal advice if you are asked to sign a cost-plus contract.
New South Wales
Building Commission NSW explains cost-plus contracts and says relevant cost-plus progress claims should be backed by receipts or other verifying documents. Its consumer guide says contracts over $20,000 need a progress-payment schedule and that the maximum deposit before work starts is 10%. Those figures are NSW-specific. The guide page identifies its last update as 10 February 2026; check the current version when you sign.
Tasmania
Tasmania’s July 2024 consumer guide describes both fixed-price and cost-plus structures. It says a fair and reasonable cost estimate and a price-calculation method should be stated for cost-plus work. It also notes that variations, actual prime-cost or provisional-sum amounts and other matters can change a fixed-price contract’s final cost.
Other states and territories
The examples above do not establish the rules in Queensland, Western Australia, South Australia, the Australian Capital Territory or the Northern Territory. Use the regulator for the place where the work occurs and check whether the project is domestic building work, a renovation, an extension or another category with different requirements.
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How to compare two builder proposals step by step
- Record the legal setting. Write down the state or territory, proposed signing date, project type and contract value. Check the regulator’s current requirements for that combination.
- Normalise the scope. Place both sets of drawings, specifications, inclusions and exclusions side by side. Mark every item one builder has priced and the other has omitted.
- List every allowance. Create a schedule of prime-cost items and provisional sums, including the amount, assumptions, evidence required, margin and treatment of savings.
- Test site-risk assumptions. Ask what each price assumes about access, demolition, rock, soil, services, asbestos, weather, approvals, temporary works and protection of adjoining property.
- Calculate the exposure. For a fixed price, model likely allowance shortfalls and requested changes. For cost-plus, model the fee on the forecast cost and a higher-cost scenario. Label scenarios as estimates, not promises.
- Read the variation clause. Confirm the form of notice, signatures, pricing method, margin, time adjustment and treatment of urgent work.
- Check payment mechanics. Match progress stages or cost claims to work completed and the applicable statutory schedule. Confirm deposit limits and supporting documents for your jurisdiction.
- Obtain independent advice. Have a qualified building-contract lawyer review unclear special conditions, substantial financial exposure or any proposed cost-plus arrangement before signing.
Practical questions to ask before signing
- Exactly which drawings, schedules and specifications form part of the contract?
- Which work, approvals, testing, connections, temporary services, cleaning and landscaping are excluded?
- For each allowance, what quantity, quality and installation cost does it cover?
- Is the builder’s margin applied to allowance adjustments, variations, subcontractor costs and taxes?
- How is a credit calculated when an allowance or provisional sum is not fully spent?
- What written approval is required before extra work or spending begins?
- Who can approve a variation, and how will an electronic approval be recorded?
- What records accompany each cost-plus claim, and how quickly must they be provided?
- How often will you receive a cost-to-complete forecast and updated completion date?
- What happens if you dispute a cost, delay a selection or do not approve a proposed change?
- Which events permit a price or time adjustment without an owner-requested variation?
- What insurance, warranty, defect and dispute provisions apply to this contract?
Common failure modes and how to prevent them
The cheapest fixed price wins on paper
A low quote may contain smaller allowances, more exclusions or less site investigation. Prevention: compare the inclusions schedule and allowance assumptions line by line, then price the gaps before choosing.
“Cost-plus” is treated as a blank cheque
Without a defined cost base, fee and evidence process, you cannot tell whether a claim is contractually recoverable. Prevention: attach a cost schedule, reporting template and approval matrix to the signed contract.
Selections are made after the allowance is exhausted
Late decisions can force rushed purchases and variations. Prevention: set selection dates, record the allowance beside each item and approve any difference before the order is placed.
Verbal changes become disputes
People remember conversations differently, especially when a change affects both cost and time. Prevention: issue a written variation with the revised price and date, and retain the signed record.
Progress claims are paid without checking
Paying an unsupported claim removes leverage and can hide budget drift. Prevention: check work completed, required receipts or other evidence, approved variations and the arithmetic of the fee before payment.
A decision test for your project
Choose fixed-price if you can answer “yes” to all three questions: Is the work defined well enough to price? Are the major selections and site risks understood? Do you value price predictability more than maximum flexibility? If one answer is “no”, first ask whether more design, investigation or allowances can resolve the uncertainty.
Consider cost-plus only if the uncertainty is genuine, the contract sets a fair calculation and evidence process, and you are prepared to manage the budget actively. If the builder cannot explain eligible costs, the margin, records or approval points in plain terms, pause rather than signing on trust.
Pre-signing checklist
- Confirm state or territory, contract date, project type and value.
- Complete plans, specifications, inclusions, exclusions and the selection schedule as far as practical.
- List every prime-cost item and provisional sum with its allowance and assumptions.
- Document actual-cost evidence, margin, credits and overspend approval for each allowance.
- Define variations, written-approval requirements, pricing and time adjustments.
- For cost-plus, define eligible direct and indirect costs, fee or margin, invoices, payment frequency and budget reporting.
- Check progress claims against work completed and the applicable statutory and contract rules.
- Obtain independent contract advice where terms are unclear, exposure is substantial or cost-plus is proposed.
Bottom line for an Australian homeowner
A well-documented fixed-price contract generally gives stronger price visibility when the project is sufficiently defined. Cost-plus is a specialised response to genuine uncertainty, not a cheaper version of a lump sum: it removes the promise of a final total and makes your record-checking and budget-control work essential. Reconcile scope, allowances, exclusions and risk allocation first, then confirm that the structure is lawful and suitable in your state or territory before signing.
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