For most home renovations in NSW, a fixed-price contract is the better choice. It gives you a defined dollar figure before work starts, puts baseline cost-overrun risk on the builder, and is generally preferred by lenders. Cost-plus contracts have a place, but it is narrower than most homeowners realise, and the risks are real.
That is our professional position at Adlington Homes, where we have been building home extensions and renovations in the Illawarra since 1972. Over that time, we have consistently seen that homeowners who sign cost-plus contracts without fully understanding the risk allocation tend to spend more than they planned.
This guide explains how both contract types work, when each makes sense, and what to watch for before you sign either one.
How Fixed-Price Contracts Work
A fixed-price contract, also called a lump sum contract, sets a defined dollar amount for a defined scope of work. The builder commits to delivering the project for that agreed sum. While this provides strong cost certainty, the contract is not entirely immune to change. Variations, latent conditions, and legislative changes can adjust the final figure. The key difference is that the baseline construction risk sits with the builder, not the homeowner.
What is locked in:
- Total contract price
- Full scope of work, documented in drawings and specifications
- Inclusions list, with every material, fixture, and finish specified
- Timeline and practical completion date
What can change it:
- Variations requested by the homeowner
- Latent conditions, meaning genuinely unforeseeable issues hidden in the existing structure
- Changes to legislation or building standards during construction
Under the Home Building Act 1989 (NSW), variations must be documented in writing and signed by both parties before the work is done. The limited exception is urgent work needed to prevent danger to people or damage to property, where written documentation follows promptly. This framework protects you from scope creep.
How Cost-Plus Contracts Work
A cost-plus contract, also called a managed contract or open-book contract, charges you the actual cost of materials and labour, plus a builder's margin. That margin is typically 15–25% of actual costs. The builder provides an estimate before work begins, but the final price depends on what the project actually costs to deliver.
What you pay:
- Actual cost of all materials, at the prices the builder pays
- Actual cost of all subcontractor labour
- Actual cost of direct expenses such as skip bins, scaffolding, and equipment hire
- Builder's margin on top, as a percentage or fixed fee
- Builder's supervision and project management, either separate or included in the margin
What is not locked in:
- The total price, because the estimate is a guide rather than a commitment
- Material costs, so you carry the impact if prices rise during construction
- Labour costs, so you pay if the work takes longer than expected
- Contingency, because surprises come out of your pocket rather than the builder's
The Risk Comparison
This is the core difference, and everything else flows from it:
| Risk Factor | Fixed-Price | Cost-Plus |
|---|---|---|
| Material cost increases | Builder's risk | Homeowner's risk |
| Labour overruns | Builder's risk | Homeowner's risk |
| Subcontractor pricing changes | Builder's risk | Homeowner's risk |
| Unforeseen site conditions | Shared through the latent conditions clause | Homeowner's risk |
| Design changes by homeowner | Homeowner pays a variation | Homeowner pays the actual cost |
| Builder inefficiency | Builder's problem | Homeowner pays for it |
| Budget certainty | High, with a locked price | Low, with an estimate only |
| Lender approval | Generally simpler | Can be more difficult |
The pattern is clear: fixed-price transfers construction risk to the builder. Cost-plus transfers it to the homeowner. The builder's margin in a cost-plus contract compensates them for management, not for risk. That is a fundamentally different arrangement.
When Fixed-Price Makes Sense for Most Renovations
Fixed-price is the right structure when:
- Your scope is clearly defined and documented in drawings
- You want budget certainty before you commit
- You are borrowing money and need a contract a lender can assess
- The project is a standard renovation, extension, knock-down rebuild, or new build
- You want to compare quotes between builders on a like-for-like basis
This covers the vast majority of residential renovation and extension work. If you have drawings, a specification, and a defined scope, there is no reason a competent builder cannot give you a fixed price.
When Cost-Plus Makes Sense in Rare Cases
Cost-plus may be appropriate when:
- The full scope genuinely cannot be determined before work starts, such as heritage restoration where you do not know what is behind the walls until they are opened
- You want maximum flexibility during construction and accept the cost uncertainty
- The project is genuinely experimental or architectural, with no precedent to price against
- You have a deep trust relationship with the builder and the capacity for full-time oversight
Under Victoria's Domestic Building Contracts Act 1995, cost-plus contracts are restricted for domestic building work. They are generally reserved for renovation, restoration, or refurbishment work where upfront pricing is impractical, or for projects in a separately permitted high-value class. NSW does not have the same legislative restriction, but the principle holds: if the work can be priced, it should be priced.
What Builders Will Not Tell You About Cost-Plus
The incentive misalignment
Under a fixed-price contract, the builder is financially incentivised to be efficient. Every day saved, every smarter procurement decision, and every well-coordinated trade sequence benefits their margin. Under cost-plus, the opposite is true. The longer a project takes and the more it costs, the higher the builder's total fee when the margin is a percentage. There is no financial incentive for the builder to be efficient.
We are not suggesting builders deliberately slow down. But when there is no financial consequence for inefficiency, it can creep in. An extra week here, a more expensive subcontractor there, and suddenly your estimate is 20–30% above what you planned.
The estimate is not a commitment
When a builder gives you a cost-plus estimate of $400,000, that is not a contractual commitment by default. It is their best guess. If the project costs $500,000, you pay $500,000. You can negotiate a guaranteed maximum price clause into a cost-plus contract, but this is not standard and must be explicitly agreed. Without one, there is no contractual mechanism to hold a builder to a cost-plus estimate in NSW.
Lender complications
Most banks and lenders prefer fixed-price contracts because they can assess the total commitment upfront. Cost-plus contracts create uncertainty because the lender does not know how much you will ultimately need to borrow. Some lenders are reluctant to approve cost-plus contracts, while others require extra contingency buffers above the estimate. Speak to your broker or lender early to understand their requirements.
Provisional Sums and Prime Cost Items
Even in fixed-price contracts, you will encounter allowances:
Provisional sums: An allowance for work that cannot be precisely priced at contract stage, such as rock excavation where the extent of rock is not known until digging begins. The final cost may be more or less than the allowance.
Prime cost items: An allowance for specific products where the homeowner has not made a final selection. If the contract carries a $5,000 prime cost allowance for tapware and you choose tapware costing $7,000, you pay the $2,000 difference.
A well-prepared fixed-price contract minimises provisional sums by doing thorough site investigation upfront, including geotechnical reports and existing-condition surveys. A poorly prepared contract loads up on provisional sums, effectively turning the project into a quasi-cost-plus arrangement with less transparency.
Our approach: We limit provisional sums to genuinely unforeseeable items. If a site investigation can answer a question before contract, we do the investigation. This costs more in the preliminary design phase, but it means the contract price is real rather than padded with uncertainty.
How to Protect Yourself Under Either Contract Type
If you are signing fixed-price
- Ensure the specification is detailed, using specific products, brands, and models rather than vague phrases such as “kitchen to builder's standard”
- Understand the variation process and require written approval with cost documented before work starts
- Check for excessive provisional sums; more than 5–10% of the total contract is a red flag
- Confirm the practical completion date and any liquidated damages clause
- Verify the builder's licence, insurance, and Home Building Compensation Fund cover
If you are signing cost-plus
- Get independent legal advice before signing
- Negotiate a guaranteed maximum price if possible
- Insist on fortnightly cost reporting with receipts and invoices
- Set clear approval thresholds for any single item above an agreed amount
- Engage an independent quantity surveyor to review costs monthly
- Plan for the likelihood that the final spend will exceed the estimate
Our Position
We quote fixed-price on every residential project once the scope is defined. We believe that is the professionally responsible approach. The preliminary design phase, where we invest in site investigation, detailed drawings, and full specifications, exists specifically so we can give you a real number instead of an estimate that becomes a moving target. You can see how this approach works across our completed projects.
Cost-plus has its place in genuinely complex heritage or restoration work where the scope is unknowable. But for home extensions, renovations, and new builds in the Illawarra, there is no reason a qualified builder cannot provide a fixed price if they do the upfront work properly.
If a builder offers you a cost-plus contract for a standard renovation, ask why. If the answer is that they have not done enough investigation to commit to a price, that is not a contract type preference. It is a preparation problem.
Frequently Asked Questions
Can I switch from cost-plus to fixed-price mid-project?
Practically, no. Once construction has started under a cost-plus arrangement, converting to fixed-price requires re-scoping the remaining work, which is difficult and costly. The contract type needs to be decided before work begins.
What percentage margin is normal for cost-plus?
Builder margins on cost-plus contracts vary. Industry commentary commonly cites ranges of 15–25% of actual costs, though the margin depends on the builder and project complexity. Some builders charge a lower percentage plus a separate project management fee. The total effective margin is what matters, not how it is labelled. Ask for a clear breakdown before signing.
Is fixed-price more expensive than cost-plus?
Often, yes, at contract signing. A fixed-price quote includes a risk premium because the builder is absorbing cost uncertainty. However, by the time a cost-plus project is complete, its actual cost can exceed what a fixed-price quote would have been. You are paying for certainty, and in our experience that certainty tends to result in better cost outcomes for the homeowner.
What if my builder says they cannot give a fixed price?
Ask why. If the scope is unclear, invest in proper design documentation first. If the site has unknowns, invest in site investigation such as geotechnical work, asbestos surveying, or an existing-structure assessment. If the builder still cannot price it after that work is done, consider whether they have the experience to deliver your project.
Are variations easier under cost-plus?
Yes, in the sense that changes do not require formal variation pricing because you are paying actual costs. But easier variations often lead to more of them, which leads to scope creep and budget blowouts. The discipline of a fixed-price variation process protects homeowners from impulse decisions during construction.
Next Steps
If you are comparing builders and contract types for a renovation in the Illawarra, we are happy to explain how our fixed-price process works and why we believe it delivers better outcomes for homeowners.
Contact us to discuss your project. We will walk you through our preliminary design process and show you how we arrive at a fixed contract price with no surprises.