Before a software project starts, client and team agree how the work will be paid for. The two main models are fixed scope, where a set price covers a defined list of work, and time and materials, where the client pays for the time actually spent. Each shares risk differently. Choosing well protects both your budget and the relationship.
This guide explains how each works in practice, what can go wrong, and how to choose or combine them.
How fixed scope works
In a fixed-scope contract, the team agrees to deliver a specific list of features for a set price and, usually, by a set date. The scope is written down in detail, often with wireframes and acceptance criteria. If the client wants something not on the list, it becomes a change request with its own price.
The appeal is certainty. The client knows the cost upfront, which helps with budgets and approvals. For small, well-understood projects, such as a website with known pages or a clearly specified integration, it works well.
The hidden costs of fixed price
Certainty has a price. Because the team carries the risk of underestimating, a fixed quote includes a buffer. If the work turns out simpler than expected, the client still pays the buffer. If it turns out harder, the team may cut corners to stay within budget, which can show up later as technical debt.
Fixed scope also makes change expensive. In software, learning during the project is normal: users react to early versions, rules turn out more complex, priorities shift. Under a fixed contract, each of those becomes a negotiation. Relationships can suffer when both sides argue about what was "in scope".
How time and materials works
With time and materials, the client pays for the hours or days the team works, at agreed rates. Work is usually organized in short cycles, each a sprint of one or two weeks, with priorities set by the client. The client can add, drop or change features at any time, and pays only for what is done.
This model fits products that will evolve, projects with open questions and ongoing development after launch. It gives flexibility and avoids paying for risk buffers.
The risks of time and materials
The risk moves to the client: if work takes longer, the cost rises. Without discipline, scope can grow quietly and budgets can drift. The model also depends on trust, since the client pays for effort rather than a fixed result.
Those risks are manageable. The key tools are a budget cap, regular demos of working software, a prioritized backlog and a clear view of spending against progress.
Side by side
- Price certainty: high with fixed scope, managed through caps with time and materials.
- Flexibility: low with fixed scope, high with time and materials.
- Who carries estimation risk: the team with fixed scope, the client with time and materials.
- Cost of changes: change requests with fixed scope, normal work with time and materials.
- Best for: small, clear projects with fixed scope, evolving products with time and materials.
Hybrid models
Many projects combine the two. The most common pattern is a fixed-price discovery phase, which produces workflows, wireframes and a detailed estimate, followed by a build under time and materials with a budget cap. The client knows the cost of discovery exactly and gets a much better estimate for the build.
Other hybrids include fixed price per phase or per release, and time and materials with a not-to-exceed amount. Each tries to balance certainty with the freedom to learn.
How to choose
Ask three questions. How clear is the scope? How likely is it to change once users see the product? How important is exact budget certainty compared with flexibility? If scope is clear and stable, and certainty matters most, fixed price can work. If you are building something new, or expect to learn as you go, time and materials with a cap usually gives better value.
Whatever you choose, a clear brief helps. See how to write a software requirements document.
Controls that protect you under either model
- Regular demos of working software, not just status reports.
- Written acceptance criteria for each feature.
- Visibility of time and cost spent against progress.
- Ownership of code and repositories from day one.
- A clear process for changes and decisions.
- The right to stop at the end of any phase.
How AI-accelerated development changes the picture
AI tools reduce the time needed for routine work, which lowers the cost of both models. Under time and materials, clients see the savings directly in fewer hours. Under fixed price, the savings may or may not be passed on, depending on how the quote was built. Ask how the team uses AI and how that is reflected in the price. See how AI changes software development cost.
An example
A company wants a customer portal. The idea is clear, but the details of permissions, document handling and the accounting integration are not. A fixed quote now would include a large buffer. Instead, the company pays a fixed price for a two-week discovery. The result is a clear scope, wireframes and an estimate range. The build then runs under time and materials with a budget cap at the top of the range, with demos every two weeks and a list of optional features to add if budget allows.
Red flags in any contract
- No written scope, assumptions or exclusions.
- Code and accounts held by the vendor.
- No regular demos or visibility of progress.
- Vague change process.
- Payment weighted heavily upfront with no milestones.
For more on evaluating teams, read how to choose a development partner and how much it costs to build a web app.
After launch
Once a product is live, most work becomes ongoing: fixes, small improvements and new features in response to users. Fixed-price contracts fit poorly here, because the work cannot be listed in advance. A monthly retainer or time and materials with a monthly budget is more common. Agree on response times for urgent issues, and keep a prioritized list so the budget goes to what matters most.
Many companies also keep a small reserve each month for security updates and dependency upgrades, which prevents the slow build-up of problems that makes later work more expensive.
In summary
Fixed price buys certainty at the cost of flexibility and a risk premium. Time and materials buys flexibility at the cost of certainty, which you manage with caps and regular reviews. For most new software products, a fixed-price discovery followed by a capped time-and-materials build gives the best balance. For small, well-defined jobs, a fixed price is simple and fair. Choose the model that matches how much you already know about what you are building.