Cost-plus vs fixed price: which one actually protects you?
Quick answer: A fixed-price contract sets one number before work starts, and the risk of it running over sits with the builder. A cost-plus contract charges actual costs plus a margin, so the risk of it running over sits with you. For a standard kitchen or bathroom renovation, where the scope is knowable in advance, fixed price protects the homeowner more often than not.
What each contract actually promises
Fixed price promises a number. Cost-plus promises a method, actual labour and material costs, plus an agreed margin on top, usually somewhere between 10% and 25% in the Australian residential market. That margin is often a little lower than a fixed-price builder's markup, because a cost-plus builder is carrying less risk, not because the job costs less overall.
That's the whole trade-off in one sentence. Fixed price moves the risk of the job running over onto the builder. Cost-plus moves it onto you.
What actually happens to the number on a cost-plus job
The margin scales with cost, not with efficiency
A builder's fee calculated as a percentage of total spend has no built-in incentive to finish faster or spend less, the fee gets bigger, not smaller, as costs rise. That's not an accusation, it's how the maths works, and it's worth understanding before signing.
Verifying costs is real, ongoing work
Checking labour hours and material invoices against what was actually done takes real time, on top of everything else living through a renovation already demands. Most homeowners don't have the spare hours to run that check properly, which is exactly the gap a cost-plus contract leaves open.
There's no ceiling unless one is written in
A cost-plus contract without an agreed maximum has no number to hold anyone to. A guaranteed maximum price can bridge the gap, capping total cost while still allowing some flexibility, but it needs to be negotiated up front, not assumed.
Fixed price vs cost-plus, at a glance
| Question | Fixed price | Cost-plus |
|---|---|---|
| Who carries the risk if costs run over? | The builder | The homeowner |
| Is the total known before work starts? | Yes, one number | No, only estimated |
| What does the builder's fee depend on? | The agreed price | A % of actual spend |
| Who checks labour hours and invoices? | Not required | The homeowner, ongoing |
| Best suited to | A knowable scope, standard kitchen or bathroom renovations | A genuinely custom or evolving architectural project |
The one case cost-plus genuinely fits
A genuinely custom or architectural project, where design decisions are still being made mid-build, is a real case for cost-plus, a fixed number can't honestly be set on a scope that isn't finished yet. A standard kitchen or bathroom renovation isn't that case. The scope is knowable before work starts, which is exactly what makes a fixed price realistic instead of a guess dressed up as one.
Fixed price only works if it's fixed both ways
The client shouldn't pay more for a surprise, and the renovator shouldn't eat a genuinely different scope for free either. A fixed-price quote that's actually fixed spells out what triggers a variation and what doesn't, in writing, up front, not as a phone call promising a free fix that turns into an invoice a couple of days later. That's the real test of whether a "fixed" price means anything.
Worth knowing
If the project is genuinely architectural, still being designed as it's built, gutted. isn't set up for that, fixed price needs a knowable scope, and this suits a builder who works cost-plus on purpose. Standard kitchen and bathroom renovations are exactly where a fixed number is realistic, and that's the job gutted. is built for.
Common questions
What's the difference between cost-plus and fixed price on a renovation?
A fixed-price contract sets one total number before work starts, and that number doesn't move unless the scope genuinely changes. A cost-plus contract charges the actual cost of labour and materials plus a builder's margin, so the final number depends on what the job actually costs to run, not what was agreed up front.
Who carries the risk on a cost-plus contract?
The homeowner does. If labour hours run long or materials cost more than expected, that cost passes straight through. The builder's margin is usually calculated as a percentage of the final cost, so there's less financial pressure on them to keep the number down than there is under a fixed price.
Is a cost-plus contract ever the right call?
Sometimes, on a genuinely custom or architectural project where the design is still evolving and a fixed number can't honestly be set yet. It's a much harder case for a standard kitchen or bathroom renovation, where the scope is knowable before work starts and a fixed price is realistic.
Why would a builder push for cost-plus instead of fixed price?
It shifts risk away from them. If a fixed-price quote would need a large contingency built in to cover unknowns, cost-plus lets a builder skip that buffer and pass the actual cost straight to the client instead, whatever it turns out to be.
Can a fixed-price quote still change after work starts?
Only through a proper variation, a genuine scope change, priced and agreed in writing before the new work happens. A fixed-price quote that's actually fixed spells out what triggers a variation and what doesn't, up front, not as a surprise once the wall is already open.
How do I check labour and material costs on a cost-plus job?
In theory, through itemised invoices and receipts checked against the work done, in practice it takes real ongoing effort most homeowners don't have time for while also living through a renovation. That verification burden is one of the real, practical costs of cost-plus that doesn't show up in the contract itself.
Does gutted. offer cost-plus contracts?
No. Every gutted. quote is fixed price, in writing, after a real site visit. Kitchen renovations run $25,000 to $55,000, bathrooms $18,000 to $40,000, and that's the number agreed before work starts, not an estimate that moves with actual costs.
Want to know exactly what a proper written quote should contain? See our guide to what should be in a renovation quote. See real completed work priced this way on our work page. Every gutted. quote follows a real site visit and a real BPC registration check, never a phone estimate. Check general contract guidance with Consumer Affairs Victoria.