A fixed-price contract protects you better. That is our position, and we will explain exactly why.
In Queenstown’s building market, where schist rock lurks under topsoil, where slopes demand engineered retaining, and where a single geotechnical surprise can add $80,000 to a project, the question of who carries the risk is not academic. It is the difference between a build that finishes on budget and one that spirals beyond what you can afford. If you are planning a custom home build in Queenstown, the contract you sign determines who absorbs the surprises.
Both contract types are legal, common, and used by reputable builders. But they allocate risk differently. Understanding that allocation is the most important financial decision you will make before signing anything.
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How a Fixed-Price Contract Works
A fixed-price contract (sometimes called “lump sum”) sets a single total price for the agreed scope of work. The builder delivers the home described in the contract documents for that price, regardless of what happens during construction.
If excavation hits unexpected rock: the builder absorbs the cost. If steel prices increase between signing and installation: the builder absorbs the cost. If the consent process takes longer than expected: the builder absorbs the holding costs. If a subcontractor’s quote comes in higher than estimated: the builder absorbs the difference.
You pay the agreed price. Full stop.
The catch: A responsible builder will only offer a genuine fixed price after thorough investigation. This means:
- Geotechnical testing (knowing what is in the ground before pricing)
- Detailed engineering (not guessing at foundation requirements)
- Quantity surveyor involvement (pricing every element, not estimating)
- Clear specifications (every material, fixture, and finish defined)
This investigation takes time and costs money upfront ($2,500 to $5,000 for a feasibility study). But it means the fixed price is based on evidence, not optimism.
How a Cost-Plus Contract Works
A cost-plus contract (also called “charge-up” or “open book”) means you pay the actual cost of labour, materials, and subcontractors, plus a percentage margin (typically 10 to 20 percent) or a fixed management fee to the builder.
The builder presents you with an estimate at the start, but this is not a guaranteed price. It is a forecast. If actual costs exceed the estimate, you pay the difference. If they come in under (rare in Queenstown), you pay less.
What you see: Monthly invoices showing every material purchase, every subcontractor payment, every hour of labour, plus the builder’s margin on top.
The appeal: Transparency. You see exactly where every dollar goes. Some buyers find this reassuring.
The risk: You carry all cost uncertainty. Every surprise, every price increase, every delay cost lands on your account. The builder has no financial incentive to control costs beyond reputation and goodwill, because their margin is a percentage of the total. Higher costs can mean higher builder earnings.
Where the Risk Actually Sits
This is the core question. Strip away the marketing language and contract jargon, and ask: who pays when something unexpected happens?
| Risk Event | Fixed Price | Cost Plus |
|---|---|---|
| Rock discovered during excavation | Builder pays | You pay |
| Material prices increase | Builder pays | You pay |
| Subcontractor requotes higher | Builder pays | You pay |
| Weather delays extend programme | Builder absorbs | You pay holding costs |
| Consent takes longer than expected | Builder absorbs | You pay extended prelims |
| Design error requires rework | Builder pays (if their error) | You pay (regardless) |
| Your scope changes (variations) | You pay (agreed variation cost) | You pay (actual cost + margin) |
The only scenario where you pay more under a fixed-price contract is if YOU change the scope. Adding a bathroom, upgrading the kitchen specification, expanding the garage. Those are variations, and they cost extra under any contract type.
But for everything that is NOT your choice (ground conditions, weather, material markets, subcontractor availability), fixed price means the builder carries it. Cost plus means you do.
Why Cost-Plus Is Risky in Queenstown Specifically
Cost-plus contracts work reasonably well in predictable environments. A flat section in Christchurch with known sandy soil, standard foundations, and easy truck access has few surprises. The estimate and the final cost tend to be close.
Queenstown is not a predictable environment.
Ground conditions vary dramatically between adjacent lots. Two sections 50 metres apart at Jacks Point can have completely different rock profiles. One might need $60,000 of excavation. The next might need $140,000. Under cost-plus, you do not know which you have until the digger hits it.
The weather is volatile. Snow, heavy rain, and freeze-thaw cycles can halt construction for weeks. Under cost-plus, you pay for the builder’s team to stand down, for site protection, and for extended programme costs.
Material supply is fragile. Everything comes by road over mountain passes. Supply disruptions are more common and more expensive than in urban centres. Under cost-plus, every supply delay and emergency freight cost hits your account.
Trades are scarce and expensive. If a key subcontractor becomes unavailable mid-project, the replacement will likely cost more. Under cost-plus, that premium is yours to pay.
We have seen cost-plus projects in Queenstown finish 30 to 50 percent over their original estimate. Not because the builder was dishonest, but because the estimate was never a guarantee. It was a guess in an unpredictable environment. For context on what builds actually cost here, see our Queenstown custom home cost guide.
The “Open Book” Illusion
Cost-plus is often marketed as “transparent” or “open book.” You see every invoice. Nothing is hidden.
This sounds reassuring until you realise: seeing the invoices does not reduce the total. You have full visibility of a cost you cannot control. It is like watching your electricity meter spin faster during a cold snap. You can see it happening. You cannot stop it.
Transparency is valuable, but it is not the same as protection. A fixed-price contract protects your budget. An open-book contract shows you your budget being exceeded.
Cost-plus advocates often position open-book accounting as the honest, fair approach. You see every invoice. You know exactly where every dollar goes. And that is genuinely useful information. But information does not prevent overruns. It just means you understand why you are paying more than you planned.
Consider this scenario: your builder’s estimate was $1.2 million. Eight months in, the open-book accounting shows actual costs of $1.45 million with three months of work remaining. You have complete visibility. You can see the rock-breaking invoice, the subcontractor requotes, the weather delay costs. You understand every line item. But your options at this point are: pay the additional $250,000+ to complete the home, or stop construction and lose everything invested so far. Transparency did not protect you. It informed you as the budget slipped away.
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What Does 30 Percent Over Budget Actually Mean?
For a Queenstown build originally estimated at $1.5 million, a 30 percent overrun means $450,000 in additional costs you did not plan for. That is not a rounding error. It is a deposit on a second home. It means going back to the bank, increasing your lending, extending your mortgage term, or making painful trade-offs on specification to claw back costs.
We have seen this happen to good people working with reputable builders. The problem is not dishonesty. It is the fundamental structure of cost-plus: the builder estimates based on available information, the ground reveals something different, and the financial consequence sits entirely with the client.
When Might Cost-Plus Actually Make Sense?
Complex renovations with unknown existing conditions. If you are opening up an existing home and genuinely do not know what is inside the walls (asbestos, non-compliant wiring, hidden rot, unknown structural modifications), fixed-price is difficult to offer because the scope cannot be defined until the work begins. Cost-plus with a robust provisional sum schedule and clear communication protocols is reasonable here.
Extremely high-end bespoke projects with a flexible budget. If your budget is genuinely unlimited (or close to it) and you want maximum creative freedom to make decisions during construction, cost-plus gives you that flexibility. You can upgrade materials mid-build, add features, change your mind about finishes, and pay the actual cost without negotiating variation orders. This suits a very small percentage of clients.
When you trust the builder completely and accept the risk consciously. Some clients have long-standing relationships with builders and are comfortable with the risk allocation. That is their choice. The key word is “consciously.” If you choose cost-plus, do so because you understand the risk and accept it, not because you were told it would be cheaper.
For most Queenstown buyers building a new home on a known section with a defined budget, fixed price after feasibility is the safer path.
How Ferguson Builders Makes Fixed Price Possible
Most builders who avoid fixed-price contracts do so because they lack the systems and discipline to price accurately before construction begins. Offering a genuine fixed price requires investment in the front end of the project that many builders are unwilling to make.
Our process:
- Preliminary planning session ($2,500 to $5,000, absorbed if you proceed). Site walkover with our quantity surveyor, geotechnical testing to establish ground conditions, slope and access assessment, services investigation (power, water, wastewater capacity and connection points), and review of any design panel or covenant requirements.
- Design development with cost modelling. Every design decision is modelled for cost impact in real time. This prevents the architect-first budget blowout where you spend $30,000 on drawings only to discover the build exceeds your budget. Our design-and-build approach means you never get a nasty surprise at pricing stage.
- Quantity surveyor cost estimate. Before we offer a fixed price, our QS prices every element: foundations (based on actual geotech data), framing, cladding, roofing, joinery, services, fit-out, landscaping, and professional fees. Nothing is left to guesswork or provisional sums.
- Fixed-price offer. Every line item priced. No provisional sums. No “TBC” items. The price includes everything agreed in the scope.
- Construction. We deliver the home for the agreed price. If costs exceed our estimate due to conditions we missed or prices that moved, we absorb the difference. That is the contract.
What About Variations Under Fixed Price?
Variations happen when YOU change the scope. Upgrading your kitchen, adding an extra bathroom, changing window sizes, expanding the garage. These are your decisions, made after the contract is signed.
Under fixed-price, every variation is quoted and approved in writing BEFORE the work happens. You see the cost, you agree to it (or you do not), and only then does it proceed. There are no surprises. You are always in control of your spend beyond the contracted price.
Under cost-plus, variations blend into the general cost stream. Changes happen, costs are recorded, and you see them on next month’s invoice. The distinction between “cost overrun” and “client variation” becomes blurred, making it harder to hold anyone accountable for budget performance.
Red Flags in Building Contracts
Watch for these warning signs regardless of which contract type you are offered:
Excessive provisional sums. If more than 10 percent of the contract value is marked as “provisional,” the builder is guessing. Foundations, excavation, and landscaping are the most common areas where builders hide uncertainty in provisional sums. A properly investigated project should have minimal provisionals.
No geotechnical investigation before pricing. In Queenstown, pricing foundations without geotech data is gambling. If your builder has not tested the ground, they cannot offer a genuine fixed price for foundations. They are either guessing (and you will pay the difference under cost-plus) or inflating the price to cover unknown risk (which costs you either way).
“Fixed price subject to…” clauses. A fixed price with extensive exclusions and conditions is not genuinely fixed. Read the conditions schedule carefully. If it lists items like “subject to ground conditions,” “subject to material price movements,” or “subject to consent conditions,” the risk is still with you.
No clear variation process. Any contract should specify exactly how variations are requested, quoted, approved, and documented. Without this, disputes about what was included in the original scope are inevitable.
Frequently Asked Questions
Is a fixed-price contract more expensive upfront than cost-plus?
The contract price may appear 5 to 10 percent higher because the builder has priced in risk contingency. But the final cost is almost always lower than cost-plus because the builder absorbs overruns rather than passing them to you. Paying a small premium for certainty is cheaper than paying no premium and absorbing unlimited risk.
What does the Construction Contracts Act say about contract types?
The Construction Contracts Act 2002 governs payment terms, progress claims, and dispute resolution for all construction contracts in New Zealand. It does not mandate one contract type over another. Both fixed-price and cost-plus are valid under the Act. The Act does, however, provide mechanisms for resolving payment disputes, which are more common under cost-plus arrangements where the scope of what is “owed” is less clearly defined.
Can I switch from cost-plus to fixed price mid-project?
Technically possible but practically difficult and usually more expensive. A builder being asked to take on risk mid-project (after some unknowns have been revealed but others have not) will price conservatively. You are better off starting with the right contract type from the outset.
Do all Queenstown builders offer fixed price?
No. Many prefer cost-plus because it eliminates their risk. Some offer “fixed price” with extensive provisional sums, which is not genuinely fixed. A contract with $150,000 in provisional sums is a cost-plus contract wearing a fixed-price label. We explore this further in our article on the danger of the lowest quote.
What protections exist under cost-plus if costs spiral?
Your main protection is the builder’s reputation and your relationship with them. The Master Builders NZ guarantee scheme provides additional assurance under qualifying contract types, covering workmanship and structural defects. However, neither reputation nor guarantee schemes cap your cost exposure. You can also set a “cost ceiling” in a cost-plus contract, but this is essentially converting it to a form of fixed price (and builders may be reluctant to agree).
What happens if the builder goes over budget on a fixed-price contract?
That is the builder’s problem to solve, not yours. They may absorb the overrun from their margin, negotiate better rates with subcontractors, find efficiencies in the programme, or accept a reduced profit on the project. Your obligation is limited to the contracted price plus any variations you have approved in writing.
Our Recommendation
If you are building in Queenstown, invest in a proper feasibility study, get a fixed-price contract based on real site data, and sleep well knowing your budget is your budget.
The alternative is cheaper to start, transparent to watch, and unpredictable to finish.
Book a feasibility session to get a genuine fixed price for your Queenstown build.
Or contact us directly to discuss your contract options and budget.