Before a single wall comes down or a slab gets poured, there’s a decision that quietly shapes your whole renovation or new build: how you’ll pay for it. In Australia, most residential building work is contracted in one of two ways, a fixed price contract or a cost-plus contract. They sound like small print, but they affect your budget certainty, your risk, and how involved you’ll be along the way.
Here’s a plain-English breakdown so you can walk into that first conversation with your builder feeling confident.
What is a fixed price contract?
Under a fixed price contract (also known as a lump sum contract), you and your builder agree on a single total for a defined scope of work. If materials get dearer or a trade takes longer than planned, that’s generally the builder’s problem to absorb, not yours.
Why homeowners like it
- Budget certainty. The total is on the table before work starts.
- Easier to compare. Lining up quotes from different builders is far simpler.
- Less paperwork. There’s no need to check invoices and receipts line by line.
What to watch for
- The price is only as good as the scope. You’re covered for what’s written in the contract and plans, and nothing more. Where the scope is fuzzy, disputes tend to follow.
- Prime cost items and provisional sums. These are placeholder budgets for things that haven’t been chosen yet (think tapware, tiles and appliances) or work that can’t be costed until it’s uncovered. Choose something pricier than the allowance and the gap is yours to cover, so check the figures are realistic.
- Variations. Any changes you make after signing, plus unexpected site conditions, will move the price. Fixed price means certainty on the agreed scope, not a promise that nothing will ever change.
What is a cost-plus contract?
A cost-plus contract works the other way around. You cover the real cost of labour, materials and subcontractors as they’re incurred, then add the builder’s fee, which is either a percentage or a set amount. The end figure isn’t known on day one; it builds up as the project moves along.
Where it can make sense
- Complex or uncertain projects. Think heritage homes, major structural changes, or older houses where nobody really knows what’s behind the walls until they’re opened up.
- Evolving designs. If you expect to tweak selections or scope along the way, cost-plus gives you more room to move.
- Visibility. You get a clear view of what’s being spent and where.
What to watch for
- Less budget certainty. With no firm ceiling, costs can creep. Ask whether there’s an estimate in the contract, and how you’ll be alerted if it looks like being exceeded.
- More hands-on for you. Expect regular cost updates and closer day-to-day contact with your builder.
- How the fee works. A percentage fee grows as costs grow, which is why plenty of homeowners lean towards a set fee. Make sure you understand the calculation.
Which one is right for your project?
There’s no one-size-fits-all answer, but this gives you a rough guide:
| Fixed price | Cost-plus | |
| Best for | Well-defined projects with finished plans | Uncertain or evolving scopes |
| Budget certainty | High (for the agreed scope) | Lower, tracked as you go |
| Your involvement | Lower | Higher |
| Main risk | Variations and underpriced allowances | Costs creeping up |
For most renovations, extensions and custom homes where plans and selections are locked in, fixed price delivers the clarity homeowners are usually after. Cost-plus is better suited to jobs with too many unknowns to price responsibly up front.
Your protections in NSW
Whichever structure you choose, home building work in NSW falls under the Home Building Act. Larger jobs need a written contract with specific details included, the deposit you can be asked to pay is limited, and home warranty insurance generally applies to eligible work. Because the thresholds and requirements get updated from time to time, confirm the current rules with NSW Fair Trading before signing, and consider having the contract reviewed independently if anything feels unclear.
What to ask your builder before you sign
- What’s in the price, and what’s left out?
- Which allowances are prime cost or provisional, and where did those numbers come from?
- What’s the process for pricing, approving and recording a variation?
- If the cost or timeline shifts, how and when will I hear about it?
- Are payments linked to finished stages of the build?
How All Innovative Building can help
Based on the Northern Beaches, All Innovative Building has spent over a decade delivering renovations, extensions and custom homes for local families. With design, council lodgement and construction all handled under one roof, plans and pricing stay in step from the very beginning, which means fewer unpleasant surprises when it’s time to talk numbers. The Opinion of Probable Cost calculator on the AIB website is also a handy way to get a feel for your budget before you commit.
If you’re about to price a project and want to talk through how best to structure it, get in touch with All Innovative Building and we’ll walk you through your options.