Fixed-price and cost-plus building contracts allocate price risk differently. A fixed price sets a contract sum for a defined scope, subject to the adjustments allowed by the contract. Cost-plus uses the actual costs defined in the contract plus an agreed fee or margin method. Neither structure is automatically cheaper or safer.
The appropriate choice depends on how complete the drawings, specifications, selections and site information are, which risks each party accepts, and how clearly the contract deals with allowances, variations, payments and records. Compare the actual contract documents rather than relying on the label alone.
The short answer for a New Zealand build
A fixed-price contract can offer clearer starting-cost certainty when the scope is well defined. It does not mean the amount can never change. Variations, provisional sums, prime-cost items, owner changes and other adjustments allowed by the contract may affect the final price.
A cost-plus contract can suit genuine uncertainty. The owner pays the recoverable costs defined in the contract plus the agreed fee method, so the final amount moves with the work. That can be useful where opening up an existing building is likely to reveal information that could not reasonably be confirmed at the start. It also means the owner needs clear records, reporting and controls.
Choose the structure by checking the scope, exclusions, allowances, site assumptions, payment terms and variation process. Obtain project-specific legal advice before signing.
Fixed-price building contracts: how they work
A fixed-price contract sets an agreed contract sum for an agreed scope of work. The price may be built from labour, materials, subcontractors, overheads, profit, allowances and other items listed in the contract. The important question is not simply whether the price is called fixed. It is what documents define the scope and what the contract allows to change it.
A fixed price is more useful when the drawings, specifications, selections, engineering information and site conditions are sufficiently settled. If an item is excluded, provisional, based on an assumption or left for a later decision, the starting price may not represent the final cost of that item.
Who absorbs an overrun?
The contract allocates the risk. A builder may carry the risk of an estimating error within the defined scope, while the owner may pay for an owner-requested change, a latent condition outside the contract assumptions or an authority requirement dealt with under the contract. Read the adjustment clauses carefully. They determine whether a cost is included, excluded, provisional or capable of becoming a variation.
There is no single percentage that can safely be applied to every project for overhead, profit or risk. Project size, complexity, procurement, site conditions and the contract terms all affect the total. Compare the complete scope, exclusions, allowances and adjustment clauses instead of relying on an assumed industry margin.
Cost-plus or charge-up contracts: how they work
Under a cost-plus arrangement, the owner pays costs the contract defines as recoverable, plus the agreed fee method. The fee may be fixed, percentage-based or calculated another way. The contract should state which labour, materials, subcontractors, overheads, GST, credits, discounts and other expenses are included or excluded.
Cost-plus does not mean that every cost is automatically recoverable. The contract should identify the records required, who can approve expenditure, how invoices and time are reported, how forecasts are updated, and how disputed amounts are handled. If the fee method is unclear, the owner may struggle to compare forecasts with actual costs.
Who absorbs an overrun?
The owner generally carries more actual-cost movement under cost-plus, subject to the controls in the contract. A realistic budget, agreed contingencies, regular reporting, early procurement and prompt decisions can help manage that movement. They do not turn an uncertain project into a fixed price.
Head-to-head comparison
| Question | Fixed-price | Cost-plus |
|---|---|---|
| How is the price formed? | An agreed contract sum for a defined scope, subject to permitted adjustments | Recoverable actual costs plus the agreed fee method |
| Fee and overhead treatment | Included in the contract sum. Check inclusions, exclusions and allowances | The contract must define the fee formula and which costs it applies to |
| Main cost risk | Allocated by the contract. Permitted adjustments may change the price | The owner generally carries more actual-cost movement, subject to contract controls |
| Strongest fit | Scope, selections and site information are sufficiently settled | Genuine unknowns remain and transparent cost reporting is accepted |
| Changes to scope | Usually require a written variation with price and time effects | May flow into actual costs, but still require clear scope and approval |
What to settle before comparing contracts
Compare like with like. For each proposal, record:
- the drawings, specifications and schedules that define the scope;
- inclusions, exclusions and owner-supplied work;
- provisional sums, prime-cost items and other allowances;
- site, access, service and existing-building assumptions;
- responsibility for design, engineering, consents and inspections;
- every mechanism that can change the price;
- for cost-plus, the recoverable-cost definition and fee calculation;
- the evidence accompanying each payment claim;
- the cost-reporting and forecast-to-complete frequency; and
- programme, delay, defects and dispute provisions.
A low headline figure is not directly comparable with a proposal containing more scope, investigation or risk. Ask what has been priced, what has been assumed and what remains to be decided.
What suits a Dunedin renovation?
Renovation work can involve information that is difficult to confirm before demolition or opening up. Existing wiring, plumbing, insulation, moisture, foundations, structure and hazardous materials may need further investigation. WorkSafe’s asbestos guidance explains relevant risks and duties.
For a project with genuine unknowns, a cost-plus or hybrid structure may allow the contract to deal with information as it becomes available. That only works when the cost definition, records, reporting and approval controls are clear. A fixed-price structure may offer more useful starting-cost certainty when investigations are complete and the scope is well defined.
Do not choose a contract structure solely because a property is old or new. Check the actual condition of the building, the quality of the available information, the design stage, the site constraints and the risk allocation in the proposed contract.
How variations and payments should work
A variation should identify the changed work, why it is required, its price or pricing method, its effect on the programme, and any design, consent or procurement impact. The contract should explain how variations are proposed, accepted, documented and paid.
Under a fixed-price contract, a variation changes the agreed scope and may affect orders, labour, sequencing and time. Under cost-plus, the changed work may flow into actual costs, but that does not remove the need for written scope and approval. In either model, confirm who may instruct and approve changes, what notices are required, and how urgent or safety-related work is handled.
Keep two payment concepts separate. First, the contract’s ordinary payment timetable should state how many payments are expected, whether they are date- or stage-based, how amounts are calculated, what evidence accompanies a claim, and when payment is due.
Second, under the Construction Contracts Act 2002, a payer may respond to a payment claim with a payment schedule. The response must be in writing, identify the payment claim and state the scheduled amount. If that amount is lower than the claim, it must explain the calculation and reasons for the difference, including any withholding. The response deadline is the time stated in the contract or, if the contract is silent, 20 working days after the payment claim is served. Notice requirements and missed deadlines can have legal consequences.
NZ legal context
For residential building work costing $30,000 or more including GST, MBIE Building Performance guidance says a written contract is mandatory. The homeowner must also receive the prescribed disclosure statement and consumer-protection checklist when the threshold is met, or when that information is requested.
The contract should state the price or pricing method, scope, responsibilities, expected timing, payment process, variation process, defect treatment and dispute process. Larger or technically complex projects may use standard forms such as NZS 3910:2023 or NZS 3916:2025, but the correct form, edition and special conditions depend on the project. Have a New Zealand construction lawyer review the proposed contract and special conditions before signing.
Frequently asked questions
Is fixed-price always cheaper than cost-plus?
No. A fixed price can offer clearer starting-cost certainty for a defined scope, but permitted adjustments may change the final amount. Cost-plus can accommodate genuine uncertainty, but the owner usually carries more actual-cost movement. Compare scope, assumptions, allowances, fees and adjustment clauses.
What is the usual builder margin on a fixed-price contract?
There is no single percentage that can safely be applied to every project. Compare the contract sum, scope, exclusions, allowances and adjustment clauses rather than assuming an industry margin.
What is a normal cost-plus management fee?
There is no universal fee. The contract should state the fee method, which costs it applies to, and how overheads, GST, credits, discounts and variations are treated.
Can I change my mind under a fixed-price contract?
You can request a change, but the builder does not have to absorb its cost. The change should be described in writing with its price or pricing method and time effect before the work proceeds, subject to the contract.
Are provisional sums the same as a fixed price?
No. A provisional sum is an allowance for work whose final scope or cost is not known at signing. Several large provisional sums reduce the certainty of the overall price.
Which contract is best for an older Dunedin home?
There is no automatic answer. A cost-plus or hybrid contract may suit genuine unknowns, while a fixed-price contract may work when investigations are thorough and the scope is well defined. Review the proposed risk allocation and controls for the specific project.
What questions should I ask before signing?
Ask which documents define the scope, what is excluded or provisional, what can change the price, how actual costs and fees are calculated, what records support each payment claim, how variations and payment schedules work, and who carries each site, design, delay and approval risk. Then obtain project-specific legal advice.
Choose the contract that fits the evidence
Choose the contract that matches the information available and the risks the parties are prepared to accept. Fixed price can offer clearer starting-cost certainty for a well-defined scope. Cost-plus can accommodate genuine uncertainty when the cost definition, records, reporting and approvals are clear. A hybrid can separate known work from defined allowances, but it should not hide major unknowns behind a headline sum.
Review the custom home building approach and the guides to home renovation costs in Dunedin and the cost to build a house in Dunedin for wider planning context. Have the proposed contract and special conditions reviewed independently before signing.
Get a free consultation
Talk through your plans and the information still outstanding before choosing a contract structure.
For a project-specific discussion, book a free consultation about the scope, information still needed and contract questions to raise with your advisers.