Fixed Price vs Cost Plus Contract in New Zealand: Which Protects Your Renovation Budget?

Last updated: 1 August 2026 · By Mossy Tariq, Founder — Property Blueprint Co.

The fixed price vs cost plus contract question gets answered badly in New Zealand, because most of the advice reaching homeowners here was written for somewhere else. The imported version treats it as a clean risk trade: a fixed price moves uncertainty onto the builder and you pay a premium for that, while cost plus keeps the uncertainty with you and costs less when nothing goes wrong. That framing is not wrong. It is incomplete, because New Zealand puts a statutory floor under both arrangements that changes what you are actually choosing between.

The floor comes from the Building Act 2004 and the regulations under it. Above $30,000 including GST, a written contract is compulsory, its contents are prescribed, and a package of disclosure has to reach you before you sign. Underneath all of it sits a set of implied warranties that follow the work rather than the paperwork. Understanding that changes the decision, because the thing most homeowners fear about cost plus — that they are signing away their protection — is not what happens here.

What follows is the operator's version of the decision: what each contract type genuinely does, what the law fixes regardless of which one you sign, and the conditions under which each is the correct choice. If you are still at the pricing stage, reading a renovation quote properly comes first, because the contract only formalises what the quote already assumed.

In New Zealand the contract type decides who carries the unknowns.
It never decides whether the builder owes you good work.

What the fixed price vs cost plus contract choice actually changes

A fixed price contract — sometimes written as a lump sum contract — states one number for a defined scope of work. The builder carries the risk that the work takes longer or costs more than allowed for. If it goes well, the builder keeps the difference. If it goes badly, the builder absorbs it, provided the scope has not changed.

That last clause is where fixed price contracts actually fail. The price is fixed against a scope, not against your renovation. Anything the scope does not cover arrives as a variation, priced after you are committed and after the alternative builders have moved on. A fixed price with a loose scope is not a fixed price. It is a starting price with a variation mechanism attached, which is why the scope of works document matters more than the contract type does.

A cost plus contract charges you the actual cost of labour and materials plus an agreed margin, usually a percentage or a fixed management fee. You carry the risk of overruns. In exchange you see the real cost of the work rather than the builder's estimate of it plus a buffer, and you are not paying a risk premium for unknowns that may never materialise.

Neither is inherently safer. What differs is who is holding the uncertainty, and how visible it is to you while the work is happening.

Does New Zealand law let you sign a cost plus contract at all

Yes, and the way the law says so is the most useful thing a New Zealand homeowner can know about this decision.

MBIE Building Performance is explicit that residential building work costing $30,000 or more including GST must be under a written contract, and that the Building (Residential Consumer Rights and Remedies) Regulations 2014 prescribe what has to be in it. The prescribed list includes the parties, the site, the dates, the expected start and completion and how delays are handled, a description of the work and materials — and this: the contract price, or the method by which the contract price will be calculated. The regulations offer a fixed hourly rate with materials invoiced separately as the worked example.

That single alternative is the whole answer. New Zealand law does not treat cost plus as a lesser arrangement to be tolerated. It treats an unwritten calculation method as the defect. A cost plus contract that names the hourly rates, names the margin percentage, names which costs the margin applies to and names how invoices will be evidenced is a fully compliant contract. A cost plus arrangement that says "charged at cost plus 15 percent" and stops there is not, because the method has not actually been stated.

Two more duties attach before signature. If the work will cost $30,000 or more including GST — or if you simply ask, at any value — the contractor must give you the prescribed checklist and a disclosure statement covering their business, qualifications and any guarantees. Failing to supply them carries a $2,000 fine. Knowingly giving false or misleading information carries a fine of up to $50,000.

What protection you keep whichever contract you sign

This is where New Zealand diverges most sharply from the imported advice, and where the fear of cost plus mostly dissolves.

Implied warranties under the Building Act 2004 are automatic. MBIE states that they apply for up to 10 years regardless of whether you have a written contract or what the contract terms are, and regardless of the cost of your building project. They cover compliance with the Building Code, good workmanship, materials being fit for purpose, and work being completed in a timely manner. A breach of an implied warranty is a breach of contract.

Alongside them sits a 12-month defect repair period. If defects in the building work emerge within 12 months of the completed build date, the builder has an obligation to fix them.

Read that against the cost plus fear and it reframes. Choosing cost plus means accepting cost uncertainty. It does not mean accepting worse workmanship protection, a shorter remedy period, or a weaker legal position on defects. Those are fixed by statute and cannot be contracted away. What cost plus genuinely exposes you to is spend you did not sanction — which is a control problem, and control problems are solvable with process.

When a fixed price is the wrong choice for a New Zealand renovation

A fixed price is correct when the scope can genuinely be defined in advance. A new kitchen in a documented 2015 house, same layout, same services, specified fittings — price that fixed and hold the builder to it.

It becomes the wrong instrument when the work is substantially discovery. Renovating a pre-2000 villa or bungalow means opening a floor and finding out what is underneath. A builder asked to fix a price on that will do one of two things: load the number with a contingency that becomes their margin if the risk never appears, or price it thin and recover through variations once you are committed. The first is expensive and honest. The second is expensive and not.

The tell is provisional sums. When a fixed price quote carries several provisional sums, prime cost allowances or "subject to inspection" lines, the builder has already told you they cannot fix the price on those items. You are being offered a fixed price on the parts that were never uncertain and a cost-plus arrangement on the parts that were, without the transparency that a properly written cost plus contract would give you. That is the arrangement to interrogate, and comparing quotes like for like is how the pattern becomes visible.

How to make a cost plus contract safe to sign

Cost plus fails when nobody defined how spending gets approved. These six terms, written into the contract before signature, convert it from an open account into a controlled one.

1. Write the calculation method out in full. Name the hourly rate for each trade, name the margin percentage, and name exactly which costs the margin applies to, because the regulations require the method and a bare percentage does not state one.
2. Set a not-to-exceed figure with a written approval trigger. Agree a total that cannot be passed without your written sign-off, so the contract has a ceiling even though the line items do not.
3. Require supplier invoices with every claim. Actual cost is only verifiable against actual evidence, and a cost plus contract without invoice substantiation is a fixed price contract where the builder picks the number afterwards.
4. Define what is inside the margin and what is not. Site supervision, scaffolding hire, waste removal and delivery charges each need to be named as either a chargeable cost or a margin-covered overhead before anyone invoices for them.
5. Fix the payment rhythm to verified progress rather than the calendar. Under the implied contract terms a progress payment is due within 20 working days of a written payment demand, so the claim needs to arrive with the evidence attached rather than ahead of it.
6. Name the Licensed Building Practitioner responsible for restricted building work. Any work essential to primary structure or weathertightness must be carried out or supervised by an LBP who issues a Record of Building Work, and verifying that person before signature is easier than after.

Every one of those is a decision that belongs before signature, which is the point in The 12-Phase System where the leverage sits. After signature you are negotiating from inside the contract instead of about it.

Which contract should you actually sign

Use the condition of the house as the deciding variable, not your appetite for risk.

Where the scope is genuinely knowable — modern construction, accessible services, no structural change, fittings specified — take the fixed price and hold the scope tight. Where the work depends on what demolition reveals, a properly written cost plus contract with a not-to-exceed figure and invoice substantiation will usually cost less than a fixed price loaded with a risk premium you cannot see and cannot audit.

The hybrid is often correct and rarely offered: fixed price the defined trades, cost plus the discovery work, and write both mechanisms into the one contract with separate approval rules. Ask for it in those words. A builder who cannot construct that contract is telling you something useful about how they run projects, under either the Building Act 2004 or the Consumer Guarantees Act.

If the cost baseline is the right first step, use the free New Zealand Renovation Cost Calculator — a trade-by-trade estimate for your specific renovation, in under 5 minutes, before any contract is put in front of you.

Frequently asked questions

Is a written contract compulsory for renovations in New Zealand?

Yes, for residential building work costing $30,000 or more including GST. MBIE Building Performance states the requirement comes from the Building Act 2004, and the Building (Residential Consumer Rights and Remedies) Regulations 2014 prescribe what the contract must contain. Below $30,000 a written contract is not compulsory but is still strongly recommended, because a signed record of price, scope and dates is what makes a dispute resolvable.

Is a cost plus contract legal in New Zealand?

Yes. The prescribed contract contents require either the contract price or the method by which the contract price will be calculated, with a fixed hourly rate and separately invoiced materials given as the example. A cost plus contract that names the rates, the margin percentage, the costs the margin applies to and how invoices are evidenced satisfies that requirement. An arrangement that states only "cost plus a percentage" does not, because the method has not been set out.

Do implied warranties still apply under a cost plus contract?

They do. MBIE states that the implied warranties under the Building Act 2004 apply for up to 10 years regardless of whether you have a written contract, what the contract terms are, or the cost of the project. They cover Building Code compliance, good workmanship, materials being fit for purpose and timely completion. Choosing cost plus changes who carries the cost risk. It does not change your workmanship protection.

What is the 12-month defect repair period?

If defects in the building work emerge within 12 months of the completed build date, the builder has an obligation to remedy them. It sits alongside the implied warranties rather than replacing them, so a defect found in year three is still covered by the warranty position even though the 12-month period has passed. Document the defects list at practical completion so the record starts on the day the clock does.

What must a builder give me before I sign a contract?

If the work will cost $30,000 or more including GST, or if you request it at any value, the contractor must supply the MBIE prescribed checklist and a disclosure statement covering their business, relevant qualifications, and any guarantees or warranties offered with their terms and exclusions. Failing to supply these carries a $2,000 fine, and knowingly providing false or misleading information carries a fine of up to $50,000.

How do I stop a cost plus contract running away on me?

Put a ceiling and an evidence rule in writing before signing. Agree a not-to-exceed figure that cannot be passed without your written approval, require supplier invoices with every progress payment claim, and define which overheads are inside the margin rather than chargeable on top. Those three terms convert an open account into a controlled one, and none of them can be added credibly once the work has started.