Cost-plus versus fixed price building contracts
Fixed price and cost-plus describe how construction is paid for. Negotiated tender describes how a builder may be selected and how the price is developed before construction. They are related commercial decisions, but they are not interchangeable.
This distinction matters because a project can use a negotiated tender and still finish with a fixed-price construction contract. It can also use a separate preliminary-services agreement for investigation or pre-construction advice without turning the construction work into cost-plus.
The right structure depends on the documents, the unknowns, the project value and Victorian law. It should be settled with the builder, architect and a construction lawyer before work begins.
Fixed-price contracts
Under a fixed-price contract, the builder agrees to deliver the defined work for the contract price, subject to the contract's rules for variations, provisional sums, prime cost items, latent conditions, delays and other adjustments.
Fixed price works best when:
- The architectural, structural and services documents are coordinated
- Selections and finishes are substantially resolved
- The site and existing conditions have been investigated
- The parties can identify and allocate the remaining risk
- The tender inclusions and exclusions are clear
The strength is price certainty for a defined scope.
The limitation is equally important. A fixed price does not make incomplete documentation complete. If the design continues to change, site conditions differ from the information available or important scope was excluded, the contract may still move through variations and adjustments.
The best fixed-price contract is supported by good documents and a realistic risk position. It is not created by forcing a builder to guess more confidently.
Cost-plus contracts
Under a cost-plus arrangement, the client pays the actual cost of the work plus the builder's agreed fee or margin. The contract should define what costs are reimbursable, what records are provided, how the fee is calculated, how budgets are reported and how variations in scope are managed.
The potential strength is transparency and flexibility where the scope cannot be fully defined.
The potential weakness is that the final cost is not fixed. The client carries more cost risk and needs disciplined reporting, approvals and forecasting.
In Victoria, cost-plus domestic building contracts are subject to restrictions. At the date of this review, Consumer Affairs Victoria states that the cost-plus method is generally not permitted for domestic building contracts below the applicable statutory threshold. Limited investigative work on an existing house may be treated differently, but a fair and reasonable estimate is still required and the remaining work must be placed under a fixed-price contract.
The law and thresholds can change. This article should link to current Consumer Affairs Victoria guidance and state clearly that it is general information, not legal advice.
Preliminary services and investigative work
Some projects need paid work before a construction price can be responsibly determined.
That may include:
- Opening up selected areas of an existing building
- Surveying concealed conditions
- Trial pits or additional geotechnical investigation
- Trade pricing and buildability reviews
- Temporary works design
- Detailed programming
- Coordination workshops
This work can be governed by a separate preliminary-services or early-contractor agreement. The agreement should define the deliverables, fee, intellectual property, insurance, termination rights and whether either party is obliged to proceed to construction.
It should not be described loosely as “starting cost-plus” unless that is genuinely the legal and commercial structure being used.
Where negotiated tender fits
Negotiated tender is the pathway used to develop the construction offer with a preferred builder.
The builder may provide open or transparent pricing during design, test major packages with trades and progressively replace allowances as the documents improve. When the scope is ready, the parties may negotiate a fixed-price contract or another permitted form suited to the project.
The important point is sequence:
- Select the procurement pathway.
- Define the pre-construction engagement.
- Develop the scope, price and risk position.
- Select and negotiate the construction contract.
Those steps should not be collapsed into one label.
Neither contract fixes poor information
Contract choice can allocate risk. It cannot remove risk created by:
- Missing architectural details
- Uncoordinated engineering and services
- Unresolved selections
- Inadequate site investigation
- Undefined interfaces between trades
- Unrealistic programs
Under fixed price, those gaps may appear as exclusions, contingencies, qualifications or variations. Under cost-plus, they may appear as rising actual costs. Under either model, the better answer is to improve the information before construction.
What clients should ask
Before agreeing to the commercial structure, ask:
- Which documents define the scope?
- Which items remain provisional?
- Who carries the risk of latent conditions?
- How are trade costs demonstrated?
- What is included in the builder's margin?
- How are preliminaries treated if the program extends?
- How are variations quoted and approved?
- What reporting will the client receive?
- Who administers the contract?
- What security, retention or guarantees apply?
- What happens if the parties do not proceed after pre-construction?
TCON's position
TCON does not treat one contract form as universally superior. A well-documented architectural home may be suited to a fixed-price contract. A project with genuine investigative uncertainty may require a carefully structured preliminary stage. A complex project may benefit from negotiated tendering before the final contract is agreed.
The commercial structure should follow the project, not the other way around.
Important note
This article is general building information. Domestic building contract requirements and insurance arrangements change. Clients and builders should obtain current project-specific legal and insurance advice before signing.