One of the first decisions in any software project — and one of the most consequential — is how you'll pay for it. Fixed price: one agreed number for an agreed scope. Time and materials: you pay for the actual time spent, at agreed rates. Each model creates different incentives, different risks, and suits different kinds of work.
This guide explains both contract models as they work in the UK market, when each one wins, the traps in each, and how to choose. Get this decision right and the commercial structure works for you; get it wrong and you'll be fighting the contract instead of building the software.
Fixed price: what it is and how it works
Fixed price means the development partner quotes one number for a defined scope of work. If it takes longer than expected, that's their problem. If it takes less time, they keep the difference. The price is fixed; the scope is fixed; the timeline is usually fixed too.
How it's structured in practice: the project starts with a discovery or specification phase (sometimes paid separately), producing a detailed scope document. The fixed quote is based on that scope. Payment is typically milestone-based — 20–30% upfront, the rest on delivery of agreed milestones. Changes go through change control: estimated separately, approved in writing, added to the price.
When fixed price wins
- Well-defined scope. You know exactly what you need — a website rebuild to an approved design, a defined integration, a feature set that's been specified in detail. If the scope document is solid, fixed price gives you budget certainty.
- Fixed budgets. Grant-funded projects, board-approved budgets, public sector work — situations where "we'll see what it costs" isn't an acceptable answer.
- Short, bounded projects. Six to twelve weeks of clearly defined work. The shorter the project, the less can go wrong with the estimate.
- Low trust or new relationship. When you haven't worked with a partner before, fixed price limits your downside while you learn how they operate.
The traps in fixed price
- The estimate includes a risk premium. The partner is absorbing the overrun risk, so they price it in — typically 15–30% above the expected cost. You're paying for certainty, which is fair, but know that you're paying for it.
- Change control becomes a battleground. "That's out of scope" is the most expensive sentence in software. Every ambiguity in the scope document becomes a negotiation, and the partner has every incentive to classify work as changes.
- Quality gets squeezed. When the price is fixed and the work runs long, the partner's margin comes from somewhere — usually testing, polish, and documentation. Fixed-price projects need rigorous acceptance criteria to prevent this.
- It punishes learning. If you discover mid-project that users need something different, changing direction costs you change-control fees and delay. Fixed price assumes you knew what you wanted at the start — which, honestly, you often didn't.
- Garbage in, garbage out. A fixed price on a vague scope is a fiction. If the specification is thin, the partner will interpret it in the cheapest way that technically meets the words — and you'll argue about the gaps for months. Our piece on software development problems covers what vague requirements do to projects.
Time and materials: what it is and how it works
Time and materials (T&M) means you pay for the actual time spent, at agreed day or hour rates, plus any materials or third-party costs. There's no fixed total — you buy the team's time, directed by you (or your product owner), typically in weekly or monthly cycles.
How it's structured in practice: agreed rates per role (e.g. senior developer £500–£800/day in the UK market, though rates vary widely by region and specialism), regular timesheets or sprint reports, and invoicing against actual time. Good T&M arrangements include budget guardrails — a "not to exceed" figure, or approval gates at spending thresholds — so it's not a blank cheque.
When time and materials wins
- Evolving or exploratory scope. You're building something new — a product, a platform, a system where requirements will emerge from user feedback. T&M lets you change direction without renegotiating the contract.
- Long-term partnerships. Ongoing development, maintenance, and continuous improvement. T&M is the natural model for a team that works with you month after month.
- Complex or uncertain work. Integrations with undocumented APIs, legacy system archaeology, R&D — work where honest estimation is impossible. Anyone giving you a fixed price for genuinely uncertain work is either padding heavily or planning to cut corners.
- You want control. T&M with an engaged product owner gives you direct control over priorities sprint by sprint. You're managing the team, not managing a contract.
The traps in time and materials
- No budget certainty. The obvious one. Without guardrails, costs can drift — especially if priorities aren't actively managed.
- It rewards slowness. The partner earns more the longer things take. This is why T&M only works with transparency: visible backlogs, sprint demos, and velocity tracking that lets you see what your money bought.
- Needs your time. T&M without an engaged client side drifts. Someone on your team must prioritise, review, and give feedback regularly — budget 5–10 hours a week minimum for a meaningful project.
- Rate shopping misses the point. A £400/day developer who ships in three days is cheaper than a £250/day developer who takes ten. Judge on output and velocity, not the day rate.
Head-to-head comparison
| Fixed price | Time and materials | |
|---|---|---|
| Budget certainty | High — one number | Low without guardrails |
| Flexibility to change | Low — changes cost extra | High — reprioritise freely |
| Best for | Defined scope, short projects | Evolving scope, long partnerships |
| Risk sits with | The supplier (priced in) | You (managed by you) |
| Requires from you | Great specification upfront | Ongoing time and decisions |
| Typical UK use | Website builds, defined integrations | Product development, ongoing work |
| Watch out for | Scope battles, squeezed quality | Cost drift, passive management |
The hybrid models worth knowing
Pure fixed price and pure T&M aren't the only options. Two hybrids are common in the UK market:
- Capped T&M (not-to-exceed). You pay for time, but the partner commits to a ceiling. It combines T&M flexibility with budget protection — the partner absorbs overruns beyond the cap. Works well when scope is mostly clear but not fully certain.
- Fixed price per phase. Discovery is fixed price, then each build phase is fixed-priced based on what was learned. You get budget certainty per phase and the ability to stop, redirect, or re-scope between phases. This is often the sweet spot for larger projects — and it's how we typically structure application development engagements.
How to choose: five questions
- How well-defined is the scope? Crystal clear and unlikely to change → fixed price. Evolving or exploratory → T&M.
- Is the budget genuinely fixed? Board-approved, grant-funded, can't-move number → fixed price (or capped T&M). Flexible budget with active management → T&M.
- How long is the engagement? Under three months with clear deliverables → fixed price. Ongoing or multi-phase → T&M or phased fixed.
- How much time can you commit? Minimal involvement wanted → fixed price with strong acceptance criteria. You can give 5–10 hours/week of product ownership → T&M.
- What's the trust level? New partner, unproven relationship → fixed price limits downside. Proven partner, working well → T&M rewards both sides fairly.
If you're still weighing whether to build at all, our custom vs off-the-shelf guide covers the decision before this one — and our project estimation guide explains what realistic estimates look like under either model.
Protecting yourself under either model
- Get the scope (or the process) in writing. Fixed price needs a detailed scope document; T&M needs a defined working process — sprint cadence, demo schedule, reporting format, and approval gates.
- Define acceptance criteria. Under fixed price, this is how you prevent quality squeezing. Under T&M, it's how you judge whether the time was well spent.
- Agree IP ownership. In the UK, without a written assignment, IP in commissioned software doesn't automatically transfer to you. Get it in the contract — under both models.
- Plan the exit. Notice periods, handover documentation, code repository access. The contract should work if the relationship ends, not just if it succeeds.
- Start small. A paid discovery phase or first milestone under either model lets you evaluate the partner before committing the full budget. Our vetting checklist covers what to look for.
The bottom line
Fixed price buys certainty at the cost of flexibility; time and materials buys flexibility at the cost of certainty. Neither is better — they're tools for different jobs. Match the model to the work: defined scope and fixed budget means fixed price; evolving product and active ownership means T&M; and when in doubt, phased fixed price gives you the best of both.
If you're scoping a project and want straight advice on structure and cost, talk to us — we'll recommend the model that fits your project, even if it's not the one that maximises our invoice.
FAQs
What is fixed price vs time and materials in software development?
Fixed price is one agreed number for a defined scope — the supplier absorbs overrun risk. Time and materials means you pay for actual time spent at agreed rates, giving flexibility to change direction but less budget certainty.
Which is better: fixed price or time and materials?
Neither is universally better. Fixed price suits well-defined, short projects with fixed budgets. Time and materials suits evolving products, long-term partnerships, and complex work where honest estimation is impossible.
How much do UK software developers charge per day?
Typical UK day rates range from around £350–£500 for mid-level developers to £500–£800+ for senior developers and specialists, varying by region and expertise. Judge on output and velocity, not just the rate — a faster senior can be cheaper overall.
What is capped time and materials?
A hybrid where you pay for time spent but the supplier commits to a maximum ceiling, absorbing any overrun beyond it. It combines T&M flexibility with budget protection, and suits projects where scope is mostly — but not fully — clear.
Who owns the IP in a UK software contract?
Under UK law, IP in commissioned software doesn't automatically transfer to the client without a written assignment. Make sure your contract explicitly assigns intellectual property to you, regardless of the pricing model.
