Fixed Price vs Charge-Up: Which Building Contract Lets a Builder Add Costs? (NZ)

sp8002
May 31
4 min read

Most "the builder charged more than the quote" disputes come down to one thing: which contract you signed. Here is what a fixed price actually fixes, how charge-up shifts the cost risk onto you, and how to tell which one you are on.

By Steve Parker · Trueworks · NZ construction contract review · 6 min

What you'll learn

What a fixed price actually fixes — and what it does not
How charge-up (cost-plus) shifts cost risk onto you
How to tell which contract you are actually on

Quick answer: A fixed-price contract fixes the price for a defined scope: the builder carries the risk of under-pricing and, usually, of material price rises, and can only add to the price through genuine variations. A charge-up (cost-plus) contract bills the actual cost of labour and materials plus a margin, so the final figure is open by design and the cost risk sits with you. Most "charged more than the quote" disputes come down to which of these you signed — and whether the extra is a variation, a fluctuation, or simply how charge-up works.

There are two main ways a NZ residential build is priced, and they allocate risk in opposite directions. Knowing which one you are on tells you, before any argument starts, who is supposed to absorb a cost that comes in higher than expected.

What a fixed price fixes — and what it does not

A fixed price fixes the contract sum for the scope described in the contract. The builder carries the risk that the work costs them more than they estimated. What a fixed price does not automatically cover is two things: changes you request (which are charged as variations), and material price rises — and the builder can only pass those on if the contract has an explicit fluctuations (rise-and-fall) clause.

So on a fixed price, an extra charge has to trace back to a genuine variation or a fluctuations clause. "It cost us more than we thought" is not, by itself, a charge you owe.

What charge-up (cost-plus) is

A charge-up contract bills the actual cost of labour and materials, plus an agreed margin. The final figure is open by design — it is not known until the work is done. Charge-up suits work where the scope genuinely cannot be defined up front (some renovations, restoration, unknown ground). The trade-off is that the cost risk sits with you, which is why good charge-up arrangements come with detailed records and, ideally, a not-to-exceed cap.

The comparison

| | Fixed price | Charge-up (cost-plus) | |---|---|---| | Price certainty | High, for the defined scope | Low, by design | | Who carries a cost overrun | The builder | You | | Material price rises | The builder, unless a fluctuations clause | Passed through to you | | How changes are charged | Through the variation procedure | Folded into actual cost | | Best suited to | Well-defined scope | Genuinely unknown or evolving scope | | Your main risk | Variations creeping up the total | An open-ended final figure |

"Estimate" vs "quote" vs "fixed price" — the wording trap

These words are not interchangeable. An estimate is an educated guess and is not a fixed price. A quote

is usually an offer to do defined work for a stated price. A document headed "fixed price" that also contains a broad rise-and-fall clause is a hybrid — read that clause closely, because it is where the certainty leaks out. Read the contract, not the cover email.

Send Trueworks your contract and the line in question. You receive a written, code-cited assessment of whether it was identified, notified, and priced the way the Building Act and your contract require — a second opinion you can put straight in front of your builder. NDA available; files NZ-hosted. → Email steve@trueworks.co.nz
or start at trueworks.co.nz

Not sure a variation or charge on your build is justified?

How to tell which you are on

Look for the signals. A stated contract sum plus a variations clause points to a fixed price. Hourly or daily labour rates, a margin percentage, and the words "actual cost" point to charge-up. If you see a fixed sum and a fluctuations clause, you are on a hybrid and the fluctuations clause decides how much cost can be passed to you.

FAQ — Fixed price vs charge-up in NZ

Can a builder charge more than a fixed-price quote? Only through genuine variations, or an explicit fluctuations clause. Ordinary cost rises and a builder's own under-estimate are generally not chargeable on a fixed price.

Is cost-plus legal in New Zealand? Yes. Both pricing models are lawful. The mandatory written-contract rule for residential work of $30,000 or more, and the implied terms, apply to both.

What is a not-to-exceed cap? A ceiling agreed on a charge-up total. It is worth negotiating if you want some certainty while still using a charge-up structure.

My contract says "estimate" — is that binding as a price? An estimate is not a fixed price. It signals the final cost may differ, so check how variations and actual costs are handled.

Which is cheaper overall? Neither inherently. A fixed price builds the builder's risk into the number; charge-up exposes you to that risk directly. The right choice depends on how well the scope is defined.

How Trueworks helps

Trueworks reads your contract first to establish which pricing model actually governs it — fixed price, charge-up, or hybrid — then tests any extra charge against the rules that flow from that. You get a written, code-cited answer on whether a charge is one you owe.

About Trueworks

Trueworks is built by Steve Parker — 20 years on the analytical side of NZ construction: variation reviews, contract advisory, and AI-augmented document analysis. It is the same defensible, code-cited read a quantity surveyor would give a variation, made available to the homeowners and trades on the receiving end of one. I answer every email personally during pilot phase.

steve@trueworks.co.nz · trueworks.co.nz