Key takeaways
- Fixed scope prices a defined output; time and materials prices team capacity and flexibility.
- Uncertainty does not disappear under a fixed price—it becomes contingency, exclusions or change requests.
- Compare proposals by assumptions, acceptance criteria and team involvement, not the total alone.
- A fixed discovery followed by a flexible build is often a sensible hybrid for new products.
The commercial model changes day-to-day product decisions. Fixed scope rewards making the agreed output predictable. Time and materials rewards changing priorities as evidence arrives. Neither is automatically safer or cheaper.
The deciding factor is uncertainty: how clearly can the team describe the users, workflows, integrations and acceptance criteria before implementation begins?
What fixed scope really fixes
A fixed-scope supplier commits to a stated set of deliverables for an agreed price and usually a target schedule. It works well for a campaign site, a known integration, a migration with a complete inventory or a first release that has already been designed and tested.
Read what sits behind the number. A responsible proposal lists assumptions, exclusions, responsibilities and the process for changes. If the brief leaves important questions unanswered, the supplier must either add contingency or assume the simplest interpretation.
- Good fit: stable requirements, few external dependencies and objective acceptance criteria.
- Warning sign: a low total paired with vague deliverables such as “admin dashboard” or “payment integration”.
- Trade-off: changing one requirement may require removing another, extending the schedule or approving a change request.
How time and materials works
You pay an agreed daily, hourly or monthly rate for the people doing the work. The team maintains a prioritised backlog and reports the time or capacity used. You can replace a planned feature with a newly discovered need without renegotiating the entire contract.
This model suits a new product, legacy-system work where the code contains surprises, and ongoing optimisation. It needs active client participation: somebody must set priorities, attend reviews and stop work that no longer justifies its cost.
| Question | Fixed scope | Time and materials |
|---|---|---|
| Budget | Known for the defined deliverables | Controlled through team size, rate and timebox |
| Change | Handled through scope trade-offs or change control | Reprioritised within available capacity |
| Client effort | Higher before signing | Continuous throughout delivery |
| Supplier risk | Estimation and delivery risk | Lower commercial risk; client owns more scope risk |
| Best evidence of progress | Accepted milestones | Working software and spend reviewed each iteration |
Compare proposals without fooling yourself
- Normalise the scope: check that authentication, content entry, migration, testing, deployment and support appear in every option.
- Read assumptions and exclusions before comparing totals.
- Ask who will do the work and how much of each role is included.
- Check the definition of done, warranty period and ownership of code and design files.
- Model a realistic change. Ask how each supplier would handle it and what it could cost.
Useful hybrid arrangements
- Fixed-price discovery, producing tested flows, architecture decisions and a delivery plan.
- A fixed first release where the workflow is settled, followed by a monthly improvement team.
- A capped time-and-materials phase with weekly reporting and a stop/go decision at the cap.
- Fixed milestones for external commitments, with a flexible backlog inside each milestone.
Whichever model you choose, insist on frequent demonstrations of working software. Commercial certainty is useful, but it cannot replace seeing progress and testing assumptions while they are still cheap to change.
Written by the Prestigious Software team
We design, build and run web platforms, SaaS products and mobile apps. About us
Our team reviews each article. Claims based on standards or research link to primary sources; examples are illustrative unless identified as client work.
