Contract type is one of the most important financial decisions in a new build. The two common structures are fixed price and cost plus. Neither is automatically right in every case, but they allocate risk differently.
What Is a Fixed Price Contract?
A fixed price contract sets the agreed build cost upfront based on defined scope and inclusions.
Typical strengths:
- better budget certainty
- easier finance planning
- clearer expectation for progress and claims
Typical watchouts:
- scope must be well defined before signing
- owner-requested changes can still create variations
- quality assumptions must be clearly documented
What Is a Cost Plus Contract?
In cost plus, you pay actual project costs plus an agreed builder margin or fee.
Typical strengths:
- flexibility for evolving scope
- transparent view of actual invoices and costs
- can suit complex or less-defined projects
Typical watchouts:
- less final price certainty
- final spend can rise if scope keeps changing
- requires disciplined oversight and decision control
The Real Difference: Who Carries Cost Risk?
- In fixed price, more cost risk is carried by the builder once scope is locked.
- In cost plus, more cost risk sits with the homeowner because actual costs flow through.
That is why documentation quality matters in both models.
When Fixed Price Often Works Better
Fixed price is often preferred when:
1. You want strong budget certainty.
2. Finance approvals depend on known total cost.
3. Scope and selections can be resolved before contract.
4. You want fewer moving parts during construction.
When Cost Plus May Be Appropriate
Cost plus may suit projects where:
1. Scope is intentionally evolving.
2. Design complexity is high and hard to lock early.
3. Homeowner wants hands-on flexibility and accepts financial variability.
4. Reporting and governance discipline are strong.
Questions Homeowners Should Ask Before Signing
- Exactly what is included and excluded?
- How are variations approved and priced?
- What reporting is provided each month?
- What assumptions in the quote are still unresolved?
- What controls prevent avoidable overspend?
Practical Advice for Most Families
For many owner-occupiers, fixed price with clear documentation is easier to manage emotionally and financially. For highly bespoke projects with changing scope, cost plus can work if governance is strict and decisions are fast.
Final Word
A good contract is less about terminology and more about clarity, controls, and communication. Choose the model that matches your risk tolerance, decision style, and project complexity.
If you are comparing contract options for an upcoming build, speak with ARC Builders and we can explain what each pathway would look like for your project.
