Fixed Price vs Cost Plus Building Contracts in Queensland
The practical difference between fixed-price and cost-plus residential building contracts, where each one suits, and the clauses to read closely.
What each contract actually commits to
A fixed-price contract sets a total price for a defined scope of work. The builder carries the risk of their own pricing being wrong, provided the scope does not change. Most residential new builds in Queensland are written this way.
A cost-plus contract charges the actual cost of labour and materials plus an agreed margin. The owner carries the pricing risk, and the final figure is not known until the work is complete.
Neither is inherently better. What matters is how well the scope can be defined before work starts.
When fixed price is the right choice
Fixed price suits new builds and any project where the design can be fully documented in advance. If the drawings, engineering, and selections are complete, there is little reason for the builder to price contingency into the number, and you get certainty.
The critical caveat is that a fixed price only covers the scope described. Provisional sums and prime cost items are estimates within the contract, not fixed amounts, and they are the most common source of unexpected cost.
Before signing, read the provisional sum and prime cost schedule closely and ask how each figure was derived. Well-researched allowances are the difference between a fixed price that holds and one that drifts.
When cost plus makes sense
Cost plus suits work where the scope genuinely cannot be known in advance — renovations to older homes where the extent of hidden damage is unknown, heritage work, or projects where the owner wants to make decisions as the job proceeds.
In those situations a fixed price would include a large risk margin to cover the unknown, and you may pay for a contingency that never eventuates. Cost plus can be cheaper in practice, but only with a builder you trust and with proper reporting.
If you use cost plus, insist on open-book invoicing, an agreed margin percentage, a written estimate updated as the job progresses, and a mechanism to pause and reassess if costs exceed a defined threshold.
Clauses worth reading carefully in either case
The variation clause: how changes are priced, who has to approve them in writing, and what happens if you disagree on a price after the work is done.
The progress payment schedule: whether payments are tied to defined stages of work completed, and whether the deposit and stage amounts sit within the limits that apply to residential building contracts in Queensland.
Delay and extension of time provisions, the defects liability period, and the dispute resolution process. Also confirm the statutory warranty insurance is in place before work commences.
If a contract is put in front of you and something is unclear, ask the builder to explain it and consider independent legal review. A builder confident in their contract will not object.
Where a fixed price stops being fixed
Two mechanisms let a fixed-price contract move: prime cost items and provisional sums. Prime cost items are allowances for products not yet selected — tapware, tiles, appliances — and you control the outcome when you choose.
Provisional sums are allowances for work whose extent cannot be determined at contract, such as site works on an uninvestigated block or repairs in a renovation. These are riskier because you do not control the outcome, and the builder's margin typically applies to any increase.
A contract with a handful of specific provisional sums is normal. One where broad allowances cover most of the site work is not a fixed price in any meaningful sense, whatever the heading says.
When cost plus genuinely suits
Cost plus makes sense where the scope really cannot be defined — heritage restoration, a complex renovation where the structural condition is unknown, or work where the client wants to make decisions as the job progresses.
It requires trust and transparency, because you are paying actual cost plus an agreed margin. Ask how costs are substantiated, how often you receive accounts, and whether there is a cap or an agreed estimate you will be notified against.
The risk transfers to you. On a fixed price the builder carries the risk of underestimating; on cost plus you do. That is not automatically bad, but it should be a deliberate choice.
Reading a Queensland building contract
For domestic building work over $20,000, Queensland Level 2 contract requirements apply, the deposit is capped at 5 per cent, and variations must be documented. Those requirements exist for your protection — insist on them being followed rather than treating them as paperwork.
Check the progress payment stages and whether they are proportionate to the work completed at each point. Front-loaded payment schedules leave you exposed if the job stalls.
Check the extension of time clause, the defects liability period, and what happens at practical completion. If anything is unclear, ask for it to be explained and consider independent legal review before signing.