Fixed-Price Building Contract Going Over Budget? Why It Happens (NZ)

Steve Parker
Jul 3
8 min read

Updated: Jul 9

Watching a fixed-price building contract going over in NZ? A fixed price only fixes the defined scope. Here is what is not fixed, and why, and how to protect yourself.

By Steve Parker · Trueworks · NZ construction estimation · 7 min

A "fixed-price" building contract fixes the price for the scope that is actually drawn and specified — not for everything you imagined the finished house to be. The gap between those two things is where the extra cost lives.

What you'll learn

Why an Auckland "fixed-price" build can climb 10–20% and still be entirely contractual
The four parts of a fixed-price contract that were never fixed: provisional sums, prime-cost sums, variations and dayworks
What to check in the contract before you sign, so the price you agree is closer to the price you pay

Quick answer: A fixed-price building contract in New Zealand fixes the price only for the clearly defined scope — the work fully drawn, specified and priced at the time you sign. It does not fix provisional sums (estimates for work not yet detailed), prime-cost sums (allowances for items you have not chosen), variations (changes to the work) or dayworks. NZ fixed-price contracts are commonly signed on incomplete design, often around 60–70% complete, so the missing detail resurfaces later as variations and provisional-sum overruns. The structural fix is to push the design closer to complete and scrutinise the allowances before you sign, not to chase the builder afterwards.

"Fixed price" fixes the scope, not your expectations

The phrase does a lot of quiet work. Most homeowners hear "fixed price" and read it as "this is the most I will pay". What it actually means is narrower: the price is fixed for the defined scope — the specific work shown on the drawings and described in the specification at the moment of signing. Anything outside that defined scope, or anything left as an estimate inside it, is not fixed.

This is not a loophole. It is how construction pricing has to work, because a builder can only put a firm number against work that has been fully designed. The problem in New Zealand is timing. Fixed-price residential contracts are routinely signed on incomplete design — frequently in the order of 60–70% complete. The remaining detail gets resolved during construction, and every time a missing decision is filled in, it arrives as a variation or a reconciled allowance. The price was honest on the day. The drawings were just not finished.

So when an Auckland build climbs 10–20% over the contract figure, the cause is usually not a dishonest builder. It is a fixed price agreed against a design that was never complete enough to fix.

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Not sure this is fair?

The four parts of a fixed-price contract that are not actually fixed

A typical contract sum is a stack of different commitments wearing the same label. Pull them apart and the "fixed" portion is smaller than it looks.

Provisional sums. These are estimates for work that is known to be needed but not yet fully designed or quantified — site works, drainage, foundations on an awkward site. A provisional sum is a placeholder. When the work is done, it is reconciled against the actual cost, and you pay the difference up or down. Ground-related provisional sums are the classic budget-breakers: ground conditions are the single most common origin of delay and variation claims in NZ, and pile depth or count overruns surface only after piling starts — one of the most expensive moments to discover bad news.

Prime-cost (PC) sums. A PC sum is an allowance for a supplied item you have not yet selected — tapware, tiles, the kitchen, light fittings, sanitaryware. The builder inserts a number so the contract balances, but you choose the actual product later. Pick above the allowance and you pay the gap, plus the builder's margin on the increase. PC sums are where "we just upgraded a few finishes" quietly becomes five figures.

Variations. Any change to the defined scope — your changes, the designer's, or work that turns out to be necessary once walls are open. Under NZ construct-only conditions, variations are valued through a set hierarchy: an agreed price first, then contract rates, then reasonable rates, then daywork. The mechanism for how these are priced and certified is set out in how NZS 3910 §14 variations are valued. Variations also carry a higher effective margin than your base contract, because once the builder is on site there is no competitive tension — the work is no longer being tendered against anyone.

Dayworks. When a variation cannot be sensibly priced as a lump sum, it is done on dayworks: actual labour hours, plant and materials, plus an agreed percentage. Reasonable in principle, but open-ended in practice. Without a tight record of hours signed off as the work happens, dayworks is the line item that drifts.

An independent Trueworks review checks a variation, progress claim, or building contract against the relevant NZ standards and the Construction Contracts Act, so you know what is fair before you commit. Most homeowners spend far less on the review than the first disputed item would cost. See how it works at trueworks.co.nz →

A second opinion before you sign or pay

Why the extra work costs more per dollar than the original build

There is a second sting beyond the missing scope: variations are simply dearer per unit than the same work priced at tender. A main contractor typically adds overhead and margin of around 10–15% to a variation. If a subcontractor does the work, that price already carries the sub's own margin of roughly 10%. Stacked up the chain, you commonly pay something like 20–30% on top of the base cost of the extra work.

That is why two builds with the same final scope can land at very different prices. The one that priced everything competitively at tender pays margin once, with tension. The one that drip-fed the same work in as variations pays stacked margin, with none.

Fixed or variable? The contract, line by line

| Component of a "fixed-price" contract | Fixed or variable? | Why | |---|---|---| | Defined scope (work fully drawn and specified) | Fixed | This is the only part the fixed price actually fixes | | Provisional sums | Variable | An estimate for not-yet-detailed work; reconciled to actual cost | | Prime-cost (PC) sums | Variable | An allowance for items you select later; you pay any difference | | Variations | Variable | Changes to scope, valued by the §14 hierarchy; carry higher margin | | Dayworks | Variable | Actual hours, plant and materials plus a percentage; open-ended | | Contingency (if any) | Variable | A buffer you may or may not spend | | Builder's margin on the base contract | Fixed | Set at tender on the defined scope |

If your contract sum is heavy on provisional and PC sums and light on fully detailed scope, you do not really have a fixed-price contract. You have an estimate with a firm-sounding cover.

What to check before you sign

You have the most leverage before signing and almost none after. Spend it here.

Push design completeness. The closer the drawings and specification are to complete, the more of the price is genuinely fixed. Ask bluntly what percentage of the design is finished. If it is around 60–70%, expect the remaining 30–40% to arrive as variations and overruns, and budget for it.
Read the provisional and PC sum schedule line by line. Total them. If the allowances are a large share of the contract sum, that share is not fixed. Ask how each provisional sum was estimated and whether the assumptions are conservative or optimistic.
Pressure-test the ground. Ask whether the foundation and site-works allowances assume a geotechnical report, and whether ground risk has been qualified ("tagged out") or carried as a firm price. Unqualified ground risk on an Auckland clay or fill site is the most likely single source of a large overrun.
Insist on a disciplined variation process. No work proceeds without a written, priced, signed variation before it starts. This keeps the §14 valuation hierarchy working in your favour and stops surprise invoices.
Get dayworks under control. Require daily timesheets signed off as the work happens, not reconstructed at the end.
Know your payment-schedule rights. Under the Construction Contracts Act 2002, the payment-claim and payment-schedule regime is strict on both sides — if you receive a payment claim and do not issue a compliant payment schedule in time, you can become liable for the full claimed amount. Understand the timeframes before the first claim lands.

This is general information, not legal advice — get advice on your specific contract.

FAQ — fixed-price building contracts in NZ

Q1: Why did my fixed price build cost more in NZ? Almost always because the fixed price was agreed against incomplete design — commonly around 60–70% complete — so the missing detail came back as variations and provisional-sum overruns. The price was fixed for the defined scope only. Anything not fully drawn, specified or selected at signing was never part of the fixed figure.

Q2: What is the difference between a provisional sum and a fixed price? A fixed price is a firm number for fully designed work. A provisional sum is an estimate for work that is needed but not yet detailed enough to price firmly. The provisional sum is reconciled against the actual cost when the work is done, so you pay the difference either way — it is a placeholder, not a cap.

Q3: What is a prime cost (PC) sum in NZ? A PC sum is an allowance the builder includes for a supplied item you have not yet chosen — tapware, tiles, a kitchen, fittings. You select the actual product later; if it costs more than the allowance, you pay the difference plus margin. PC sums are a common reason finish selections push a build over budget.

Q4: Can a builder charge more than a fixed-price contract? Yes, legitimately, for work outside the defined scope. Variations, provisional-sum reconciliations, PC-sum upgrades and dayworks all sit on top of the fixed figure. What a builder generally cannot do is unilaterally re-price the defined scope, or bill for variations that were never instructed and agreed in writing.

Q5: How do I stop a fixed-price contract going over budget? Front-load the work before signing: push the design as close to complete as you can, scrutinise the provisional and PC sum schedule, qualify ground risk, and lock in a written-before-work variation process. Most overruns are decided at signing, not on site — the contract you sign sets how much can move.

Get an independent review before you pay or sign

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About Trueworks

Trueworks is built by Steve Parker — 20 years on the analytical side of NZ construction. Variation reviews, contract advisory, programme review, and AI-augmented document workflows. Trueworks gives homeowners and builders the same defensible, independent analysis a developer's quantity surveyor would run — in plain English, at a price and pace that makes sense for a single project.

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