Fixed Price vs Cost Plus: What’s the Right Contract for Your Renovation?
Fixed Price vs Cost Plus: What’s the Right Contract for Your Renovation?
There are many decisions to make when planning a home renovation or new build — and choosing the right builder is one of the most important.
But just as important as who you build with is how the project is structured.
In Australia, two contract types are commonly used in residential construction:
Fixed Price (Lump Sum)
and
Cost Plus
They are not interchangeable — and neither is automatically “better.”
They simply allocate risk, responsibility, and financial control differently.
Understanding how each works can help you choose the structure that aligns with your project, your budget, and your appetite for involvement.
What Is a Fixed Price Contract?
Also known as a Lump Sum Contract, a Fixed Price contract is where the builder agrees to complete the agreed scope of works for a set dollar amount.
This means:
You know the contract sum upfront.
The builder carries the financial risk for cost overruns — provided the scope does not change.
Most banks require this structure for construction loans.
It offers greater cost certainty and predictability.
However, “fixed” does not mean immune to change.
Variations can still occur due to:
Changes in design
Unforeseen site conditions
Prime Cost (PC) items
Provisional Sums (PS)
If documentation and selections are incomplete at contract stage, risk can quietly shift back to the homeowner through allowances and variations.
The contract is only as clear as the documents behind it.
What Is a Cost Plus Contract?
Under a Cost Plus contract, the builder is paid for:
Actual labour and material costs
+
An agreed margin (to cover overhead and profit)
In this structure:
The homeowner carries the financial risk for cost increases.
The final project cost is not fixed upfront.
Costs are tracked and invoiced as work progresses.
This structure can offer:
Greater transparency
More flexibility
Suitability for complex or evolving projects
But it also requires:
Active client involvement
Comfort with variable totals
Ongoing cost monitoring
Cost Plus is not automatically cheaper — it simply distributes risk differently.
Where People Get Confused
Many homeowners assume:
Fixed Price = no surprises
Cost Plus = more expensive
Neither assumption is automatically true.
The real difference lies in how complete your documentation and selections are before construction begins.
If your plans, finishes, fixtures, joinery details, and specifications are largely resolved before contract signing, both contract types can function smoothly.
If they are vague or evolving, both contract types can experience cost movement.
The contract structure doesn’t create budget blowouts.
Unresolved decisions often do.
When Might Each Suit?
A Fixed Price Contract may suit you if:
You require bank finance
You want maximum cost certainty
Your selections are largely finalised
You prefer the builder to carry financial risk
A Cost Plus Contract may suit you if:
The project is highly bespoke or evolving
You value flexibility during construction
You’re comfortable reviewing costs regularly
You want greater visibility into actual expenditure
The Bigger Picture
Regardless of contract type, the most effective way to minimise financial stress during construction is clarity before signing.
The more decisions made upfront — layouts, joinery, lighting, materials, fixtures — the fewer assumptions are left inside the contract.
Contract structure matters.
But preparation matters more.
If you’re unsure which contract structure suits your project, it’s worth discussing it early — before drawings are finalised and pricing begins.