You have finalized your architectural plans, compared your builder quotes, and you are finally ready to sign on the dotted line. But then the builder hands you the contract, and you have to make a massive, high-stakes decision:
Should we sign a Fixed-Price Contract, or should we build under a Cost-Plus (Charge-Up) Contract?
Here is the direct answer: For 90% of residential homeowners in New Zealand, a Fixed-Price Contract is the safer option because it provides strict budget certainty and is highly preferred by retail banks for construction loans. However, on highly complex renovations, or during periods of falling material prices, a Cost-Plus Contract can actually result in a cheaper, more transparent build—provided you have a highly trustworthy builder and a healthy cash contingency.
At Builders Near Me NZ, we connect homeowners across the country with verified, highly rated construction companies. We review complex contracts daily, and we know that choosing the wrong contract type is the single biggest cause of financial distress during construction.
In this comprehensive guide, we will break down exactly how these contracts work under New Zealand law, compare their pros and cons, explain why banks view them differently, and show you how to choose the safest option for your specific project.
What is a Fixed-Price building contract?
A Fixed-Price Contract (often based on standard Registered Master Builders or Certified Builders templates) is a legal agreement where the builder agrees to complete a clearly defined scope of work for a set, predetermined price.
The Pricing Mechanism: The builder calculates every single cost (labour, materials, sub-trades) based on your architectural plans, adds their profit margin, and gives you one final number.
The Risk Premium: Because the builder is carrying the financial risk of any material price spikes or labour overruns, they will bake a "Risk Premium" into the price. If the timber costs 10% more halfway through the build, the builder absorbs the loss, not you. However, if timber prices drop, the builder pockets the extra profit.
The Catch: "Fixed" does not mean a guaranteed final price. The contract price is only fixed for the exact plans provided. If you make changes, or if the builder hits unforeseen site issues, the price will increase via formal contract variations.
To understand how builders calculate these base rates, read our comprehensive guide to new home builders' costs.
What is a Cost-Plus (Charge-Up) building contract?
A Cost-Plus Contract (often called a Charge-Up or Time and Materials contract) is an agreement where you pay the builder for the actual wholesale cost of the materials and the exact hours of labour used, plus an agreed-upon margin to cover their overheads and profit.
The Pricing Mechanism: The builder has no incentive to build a "risk premium" into the quote. Every month, they will present you with an itemised invoice showing exactly what they paid for timber, concrete, and subcontractors, and then add their agreed margin (typically 10% to 15%).
The Risk Allocation: You, the homeowner, carry 100% of the financial risk. If the weather is terrible and the carpenters take an extra three weeks to stand the frame, you pay for those extra hours.
The Catch: There is no guaranteed maximum price. The "estimate" the builder gives you upfront is simply a non-binding guess.
To learn more about standard trade rates, read our detailed guide on how much renovation builders charge per hour.
Direct Comparison: Fixed-Price vs. Cost-Plus
To help you visualise how these contracts shift the financial risk, here is a direct, side-by-side comparison of how they operate under New Zealand building conditions:
Feature | Fixed-Price Contract | Cost-Plus (Charge-Up) Contract |
|---|---|---|
Who carries the risk of material price hikes? | The Builder. | The Homeowner. |
Who carries the risk of labour delays? | The Builder. | The Homeowner. |
Are the wholesale material invoices transparent? | No. The pricing is lumped into a total sum. | Yes. You have a legal right to inspect all invoices. |
How do banks view this contract? | Highly preferred. Easy to secure lending. | Disliked. Requires massive cash equity or a 20%+ contingency. |
Best suited for: | Standard new builds on flat sites with high-interest mortgages. | Complex renovations, architectural builds, or experienced developers. |
Why do banks hate Cost-Plus contracts?
If you are planning to finance your build using a bank construction loan, trying to secure lending for a Cost-Plus contract is exceptionally difficult.
New Zealand retail banks (such as ANZ, ASB, Westpac, and BNZ) are highly risk-averse. They want to know exactly what their maximum financial exposure is before they approve a loan.
With a Fixed-Price Contract: The bank knows that if they lend you $500,000, the house will be finished to a habitable standard (Practical Completion) for that price.
With a Cost-Plus Contract: There is no ceiling. If the project runs out of money at the GIB-stopping phase, the bank is left holding an unfinished, un-sellable asset.
If a bank does agree to fund a Cost-Plus contract, they will typically demand that you hold a massive cash buffer—often 20% to 30% of the estimated build cost—in reserve, whereas a fixed-price contract usually only requires a 5% to 10% cash contingency.
Case Study: The $500,000 Build Dilemma
Let’s look at a real-world scenario of two families building identical 150m² homes on adjacent sections in Christchurch.
Family A: Signed a Fixed-Price Contract ($500,000)
The Build: During construction, a massive national timber shortage struck. Framing timber prices jumped by 15%, and wet weather delayed the framing phase by three weeks.
The Outcome: Because they had a fixed-price contract, the builder had to absorb the $15,000 material price hike and the extra $8,000 in labour hours. Family A paid exactly $500,000 at handover (excluding a few minor design variations they requested).
Family B: Signed a Cost-Plus Contract (Estimated at $470,000)
The Build: Family B chose a Cost-Plus contract because the builder offered a cheaper upfront estimate ($470k vs Family A's $500k) because they didn't include a risk premium. They faced the exact same timber shortage and weather delays.
The Outcome: Because they carried the risk, Family B had to pay the actual cost of the expensive timber and the extra builder labour hours. Their final build cost was $515,000—meaning their "cheaper" contract actually cost them $15,000 more than their neighbour's fixed-price contract.
What does NZ Law say about building contracts?
In New Zealand, residential building contracts are governed by the Building Act 2004 and the Consumer Guarantees Act (CGA).
Under the New Zealand Building Act 2004, a written contract is legally mandatory for any residential building project costing $30,000 or more (including GST).
Furthermore, under the Consumer Guarantees Act, if you are building under a Cost-Plus contract where no flat price was agreed upon, the builder is legally required to charge a "reasonable price." What is reasonable is dictated by standard market rates for labour and materials in your specific region.
4 Steps to choose the safest contract for your project
Before you sign any agreement, follow this decision framework to protect your investment:
Step 1: Evaluate the Soil and Site
If you are building on a flat, geotechnically stable section with a detailed soil report, a Fixed-Price contract is highly safe. If you are renovating a 100-year-old Villa with unknown structural integrity behind the walls, a Cost-Plus contract is often fairer—but you must budget a minimum 20% building contingency budget.
Step 2: Check Your Bank Lending Requirements
Speak to your mortgage broker early. If you do not have significant cash equity and rely heavily on a high-LVR construction loan, you will likely have no choice but to sign a Fixed-Price contract.
Step 3: Interrogate the Exclusions and PC Sums
A "Fixed-Price" contract is only safe if it is comprehensive. Ensure you read our guide on avoiding hidden costs in builder quotes to ensure the builder hasn't left out essential siteworks just to make their fixed price look cheap.
Step 4: Have a Lawyer Review the Contract
Never sign a building contract—whether Fixed-Price or Cost-Plus—without a qualified property lawyer reviewing it. They will check the critical clauses regarding retentions, liquidated damages, and variation margins. Learn what to look for in our guide to decoding building contract terms.
Ready to find a trustworthy building partner?
The best builds are built on trust. Whether you choose a fixed-price or a cost-plus contract, you need a builder registered with an independent body like the Registered Master Builders Association or New Zealand Certified Builders (NZCB).
Compare top-rated, vetted builders in your specific area through Builders Near Me NZ: