“Fixed price protects you from cost uncertainty. Time and materials protects you from scope rigidity. The right choice depends on how well you know what you want.”
The trade-off in one sentenceWhen you receive a software development proposal, one of the first decisions is whether to use a fixed-price contract or a time and materials arrangement. Both are legitimate structures. Both can work well. Both can go badly wrong in the wrong context.
This is not a decision that should be made by the agency alone — it should reflect the nature of your project and how well the scope is defined at the time of signing. Here is how to think about it.
How Fixed-Price Contracts Work
In a fixed-price contract, the agency agrees to deliver a defined scope of work for a defined total cost. If the work takes them longer than expected, the extra cost is theirs to absorb. If it takes less time, they keep the saving. The price you agreed is the price you pay, provided the scope does not change.
The critical word is scope. Fixed-price contracts require a very well-defined scope document before work starts, because any change to the scope — even a small one — typically triggers a formal change request with additional cost. If your scope evolves during the project (which it almost always does), fixed-price contracts can quickly become expensive.
How Time and Materials Works
In a time and materials arrangement, you pay for the actual time the team spends on your project, typically billed weekly or fortnightly at a defined daily or hourly rate. The scope can evolve as you learn more about what users actually need. There is no change request process because change is the expected operating mode.
The risk is that the total cost is not known upfront. Without active management of scope and velocity, time and materials projects can run significantly over initial estimates. The protection is a milestone structure alongside the hourly billing, where you agree on what will be delivered in each two-to-four week sprint before it starts.
Time and Materials
- Pay for actual time spent, not a pre-agreed total
- Scope can evolve as you learn from users
- Requires closer management and regular sprint reviews
- Works best for exploratory or iterative products
- You carry the cost risk if scope expands significantly
The Hybrid Approach
Many well-run agencies offer a hybrid structure: a fixed-price discovery phase that produces a detailed scope document, followed by time and materials development against that agreed scope, with change requests required for anything outside it. This is often the most sensible approach because it sequences the uncertainty appropriately — you pay to reduce scope ambiguity before committing to a development cost.
A discovery phase typically takes two to four weeks and costs $3,000 to $15,000 depending on project complexity. It produces wireframes, a technical architecture document, a detailed feature specification, and a development estimate based on that specification rather than a vague brief. The estimate from a discovery phase is significantly more reliable than the estimate from a 30-minute sales call.
Which to Choose for Your Project
- Choose fixed price if: you have a very detailed specification, the scope is stable, and you are more worried about cost than flexibility
- Choose time and materials if: your requirements will evolve, this is a long-term product relationship, or you are building something with significant uncertainty
- Choose hybrid if: you want a reliable estimate but your current brief is too vague to produce one — invest in a discovery phase first
- Never choose fixed price if: your requirements are vague, you have no existing specification, or you know you will want to make changes during the build
The most expensive outcome in software development is a fixed-price contract on a vague scope. The agency will either build what they think you meant (which is not what you wanted), or they will hit you with change requests that add up to more than a time and materials arrangement would have cost. Define the scope first, then choose the contract structure that fits it.
