Fixed Price vs Cost-Plus Contracts: Which Is
Better for Your New Home in Christchurch?
Fixed Price vs Cost-Plus Contracts: Which Is Better for Your New Home in Christchurch?
Choosing how you pay your builder is one of the biggest financial decisions in any newhome project. The two most common options in New Zealand are the fixed price contractand the cost-plus contract, and each shifts risk between you and your builder in a verydifferent way.At FACL Construction NZ, we help Christchurch homeowners build high-quality customhomes, and the contract conversation always comes up early. In this guide, we explainboth contract types in plain language, compare them side by side, and help you decidewhich one suits your design, budget and comfort with risk
What Is a Fixed Price Building Contract?
A fixed price contract (also called a lump sum contract) sets one agreed total price for thework described in your plans and specifications. The builder commits to delivering thatscope for that amount.
How a Fixed Price Contract Works
- You and your builder agree on detailed plans, specifications and a finishes schedule first.
- The builder prices the whole job, including their margin and an allowance for risk.
- You pay in progress payments linked to stages such as foundations, framing, lock-up and completion.
- Any change to the agreed scope is handled as a written variation, priced before thework starts
Advantages of a Fixed Price Contract
- Budget certainty: you know your target cost from day one, which makes lenderapproval easier.
- Lower financial stress: the builder carries the risk of normal cost increases within theagreed scope.
- Easy quote comparison: quotes based on the same plans can be compared like forlike
Disadvantages of a Fixed Price Contract
- You need complete plans first. Half-finished designs lead to guesses, and guesseslead to disputes.
- The price may include a risk buffer, so it can look higher on paper.
- Variations and allowances can add up. Changing your mind or choosing tiles and
- appliances above the allowance increases the final cost
What Is a Cost-Plus Building Contract?
In a cost-plus contract, you pay the actual cost of labour, materials and subcontractors, plus an agreed builder’s margin. That margin is either a percentage of costs or a fixed management fee.
How a Cost-Plus Contract Works
The builder gives you an estimated price, not a fixed one.
You receive supplier and subcontractor bills with a clear cost breakdown.
The builder adds an agreed fee to the total cost.
Some contracts set a maximum price to help control costs.
Advantages of a Cost-Plus Contract
- Flexibility: it suits designs that will evolve while you build.
- Transparency: you see where every dollar goes.
- No built-in risk buffer: if costs come in under the estimate, you keep the saving
Disadvantages of a Cost-Plus Contract
- Open-ended cost risk: if prices rise or problems appear, you pay for them.
- Harder budgeting: lenders may ask for extra evidence before approving finance.
- More oversight: you will need to review invoices and make decisions quickly
Fixed Price vs Cost-Plus: Quick Comparison
Factor | Fixed Price | Cost-Plus |
|---|---|---|
Who carries cost risk | Mostly the builder | Mostly the homeowner |
Budget certainty | High, if the scope is clear | Lower, based on an estimate |
Best for | Fully designed homes | Complex or changing designs |
Flexibility | Low, changes may cost extra | High, changes are easier |
Cost transparency | Moderate | High |
Plans needed before pricing | Complete plans and specifications | Partial plans may be enough |
Lender comfort | Usually easier | Often requires more details |
Which Contract Suits Your New Home?
There is no universal winner. The right choice depends on how finished your design is, howmuch risk you can carry, and what your lender requires.
Choose a Fixed Price Contract If
- Your plans, engineering and specifications are complete.
- You are a first home builder who wants predictable costs.
- Your lender needs a firm contract price.
Choose a Cost-Plus Contract If
- Your design is complex, unusual or still developing.
- The site has unknowns, such as difficult ground or existing structures.
- You want full cost visibility and have a contingency fund
Consider a Hybrid Approach
Many homeowners get the best result from a middle path: a fixed price with realistic allowances, a cost-plus contract with a guaranteed maximum price, or a staged contract where site work and foundations are cost-plus and the build above ground is fixed price once the unknowns are settled.
Christchurch Factors That Affect Your Choice
Building in Canterbury has its own conditions, and they influence which contract is safer.
- Ground conditions: foundation requirements vary with soil and land category, so complete a geotechnical report early. Read our guide to house foundations in Christchurch for more detail.
- Weather: cold and wet months can slow progress, so ask how weather delays affect cost and completion dates.
- Material and labour prices: check who carries the risk of price rises and whethersupplier quotes are locked in.
What NZ Law Says About Building Contracts
Under the Building Act 2004, residential building work of $30,000 or more (including GST) needs a written contract, and the builder must give you disclosure information before you sign. Implied warranties also apply to the work and materials. Rules can change, so confirm current requirements with MBIE or your lawyer. This article is general information, not legal advice.
6 Things to Check Before You Sign
Whichever structure you choose, read the fine print. Look for these items:
- Clear scope of work, with plans, specifications and a finishes schedule attached.
- Provisional sums and prime cost allowances, and whether they are realistic for your
taste.
- A variation process that needs written approval and a price before work starts.
- A payment schedule linked to completed stages.
- Start and completion dates, including how weather delays are treated.
- Warranties, insurance and defects period, so you know who to call after handover.
See our guide to the hidden costs of building a home for more budget traps.
FrequentlyAsked Questions
Is a fixed price contract always cheaper?
Not always. It gives you certainty, but the price may include a risk allowance. A cost-plus contract can finish lower if everything runs smoothly, or higher if problems appear
Can a fixed price contract still go over budget?
Yes. Variations, changed selections and underestimated allowances can all raise the final cost.
Which contract is better for first-time builders?
Most first-time builders prefer fixed price, or cost-plus with a guaranteed maximum price, because the total cost is easier to plan and finance
Final Thoughts
The choice between fixed price and cost-plus is really a choice between certainty and flexibility. If your plans are complete and you want a firm number, fixed price is usuallythe safer path. If your project is complex or evolving, cost-plus with a price cap can workwell. Whatever you choose, complete plans, honest communication and a builder you trustmatter most. Not sure how to pick? Our guide on how to choose the righ inChristchurch can help
Contact Us
Ready to talk about your new home? FACL Construction NZ will help you find thecontract that fits your design, budget and comfort level. Explore our residential new build services and new build process, then contact our team for a no-pressure conversation.
Phone: +64 3 390 3225
Email: hello@facl.co.nz
Address: 100 Sir John Mckenzie Avenue, Yaldhurst, Christchurch 8042, NZ
Website: facl.co.nz