Choosing the right type of building contract is one of the most important decisions you'll make before starting a build or major renovation in New Zealand. A fixed-price contract locks in a set price for the work, while a cost-plus contract means you pay the builder's actual costs plus an agreed margin. Both have genuine advantages — and real risks — depending on your project.
What Is a Fixed-Price Contract?
With a fixed-price contract (sometimes called a lump-sum contract), your builder agrees to complete a defined scope of work for a set dollar amount. If their costs come in higher than expected, that's generally their problem — not yours.
Key features:
- The total price is agreed upfront
- Variations (changes to scope) are priced separately and added via a formal variation order
- You have certainty over your budget from day one
- The builder carries most of the financial risk
Fixed-price contracts work best when the scope of work is clearly defined, plans and specifications are finalised, and there are few unknowns. They're common for new builds where detailed architectural drawings and a full specification exist before work begins.
Watch out for: Builders pricing in a contingency to protect themselves, which can mean you pay a premium for that certainty. Also, if the scope isn't crystal clear, disputes over what's included can arise.
What Is a Cost-Plus Contract?
With a cost-plus contract, you pay the builder's actual costs — labour, materials, subcontractors, plant hire — plus an agreed fee. That fee is either a fixed management fee or a percentage of costs (commonly 10–20%, though this varies widely).
Key features:
- You see the actual invoices and receipts
- The builder's margin is transparent
- The final cost is unknown until the project is complete
- You carry most of the financial risk
Cost-plus suits projects where the scope is hard to define upfront — think complex renovations, heritage restorations, or builds where ground conditions are uncertain. It can also suit homeowners who want to be closely involved in material selections as the project progresses.
Watch out for: Without a maximum price cap, costs can blow out significantly. Always negotiate a guaranteed maximum price (GMP) clause if you go cost-plus, so there's a ceiling on what you'll pay.
Side-by-Side Comparison
| Feature | Fixed-Price | Cost-Plus |
|---|---|---|
| Budget certainty | High | Low |
| Who carries cost risk | Builder | Homeowner |
| Transparency of costs | Lower | Higher |
| Best for | Well-defined scope | Complex/uncertain scope |
| Flexibility for changes | Lower (variations costly) | Higher |
| Builder incentive to be efficient | High | Lower (unless GMP applies) |
NZ Legal Requirements for Building Contracts
In New Zealand, the MBIE Building Performance guidelines strongly recommend having a written contract for any significant building work. The Construction Contracts Act 2002 applies to residential building work and sets out important rights around payment claims, payment schedules, and dispute resolution — regardless of which contract type you use.
Your contract should clearly state:
- The scope of work and specifications
- The contract price or how costs will be calculated
- How variations will be handled and priced
- A payment schedule
- Practical completion and defects liability provisions
- What happens if there are disputes
BRANZ and Consumer NZ both offer useful guidance on what to look for in a building contract before you sign.
Building Consents and Your Contract Type
Regardless of contract type, if your project requires a building consent, this must be obtained before work begins. Your builder or a Licensed Building Practitioner (LBP) must carry out and supervise restricted building work — this includes structural work, weathertightness work, and other defined categories. Always confirm consent requirements with your local council, as some minor work may be exempt under Schedule 1 of the Building Act 2004, but the rules are specific.
Tips for Protecting Yourself
On a fixed-price contract:
- Make sure the scope and specifications are as detailed as possible before signing
- Understand exactly what is and isn't included
- Agree on a clear process for pricing variations before work starts
On a cost-plus contract:
- Negotiate a guaranteed maximum price wherever possible
- Ask for regular cost reports (weekly or fortnightly)
- Clarify exactly what counts as a reimbursable cost and what's covered by the builder's fee
- Keep a contingency fund of at least 10–15% of the estimated budget
When to Call a Professional
Before signing any building contract, it's worth getting independent advice. A quantity surveyor can review cost-plus estimates and check that rates are reasonable. A construction lawyer can review contract terms and flag anything unusual. For the building work itself, always use a Licensed Building Practitioner for restricted building work — this is a legal requirement in New Zealand, not just good practice.
If you're looking for a qualified, trustworthy builder in your area, you can search for local professionals on tradedirectory.co.nz.
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