Fixed Price vs Time and Material: Choosing the Right Development Contract

When you commission a website, app or piece of custom software, the contract model you choose shapes almost everything that follows:

How much you pay, who carries the risk when things change, how much flexibility you have and even how the development team behaves. The two most common options are fixed price, where you agree a set price for a defined scope, and time and materials, where you pay for the actual time spent at agreed rates.

The short answer is that fixed price works best when the scope is small, well defined and unlikely to change, and when budget certainty matters more than flexibility. Time and materials works best when requirements are still evolving, when you expect to learn and adjust as the product takes shape, or when the work is ongoing. Many successful projects use a hybrid: a fixed-price discovery phase to define the work, followed by time and materials development in short, capped increments, or fixed prices for individual milestones.

The common assumption that fixed price is safer and cheaper is often wrong. A fixed price does not remove risk. It moves it to the supplier, who will charge for carrying it, and it can make every change a negotiation. Time and materials does not mean an open cheque either, if it is managed well. This guide explains how each model works, the real trade-offs, when each fits, the hybrid options in between, the contract terms that protect you and how to manage a project under each approach.

How each pricing model works

Fixed price

In a fixed-price contract, the client and supplier agree on a defined scope of work, a delivery timeline and a total price before development starts. The supplier commits to delivering that scope for that price. If the work takes longer than expected, the supplier absorbs the extra cost. If it takes less, the supplier keeps the difference.

To make this work, the scope must be documented in detail, usually in a specification or statement of work describing features, screens, integrations, acceptance criteria and exclusions. Payments are often tied to milestones, such as design approval, completion of key features and final acceptance. Any change to the agreed scope goes through a formal change request process, with its own price and impact on the timeline.

Time and materials

In a time and materials (T&M) contract, the client pays for the actual time spent by the team at agreed hourly or daily rates, plus any agreed expenses such as third-party licences or hosting. The scope can change as the project progresses. Priorities are typically set and adjusted regularly, often in short iterations, and the client pays for the work done.

T&M contracts usually include an estimate and a budget, but these are forecasts rather than commitments. The client carries more of the cost risk, while gaining flexibility and control over what is built and in what order.

Who carries the risk

The core difference is not price, but who carries the risk of uncertainty. Under fixed price, the supplier carries the risk that the work takes longer than estimated, and the client carries the risk that the agreed scope turns out not to be what they really needed. Under T&M, the client carries the risk of cost and time overruns, but can change direction whenever new information emerges.

The real trade-offs between the two

Comparing the models on headline price alone misses most of what matters. These are the trade-offs that tend to decide whether a project goes well.

Cost and the risk premium

A fixed price is not simply the supplier's best estimate. Because the supplier carries the risk of overruns, a sensible supplier adds a margin, sometimes called a risk premium or contingency, to cover the uncertainty. The less clearly defined the scope, the larger that margin needs to be. If everything goes to plan, the client pays more than the work actually cost. If the supplier underestimated and the margin is not enough, the supplier may cut corners, push back hard on anything that looks like a change or, in the worst cases, walk away.

Under T&M, the client pays for actual effort without a built-in risk margin. When a project is well managed, this is often cheaper overall. When it is poorly managed, with unclear priorities and constant changes of direction, costs can grow well beyond the original estimate.

Software projects are genuinely hard to estimate. Research by Bent Flyvbjerg and Alexander Budzier, published in the Harvard Business Review in 2011, analysed a large sample of IT projects and found that while the average overrun was modest, about one in six had cost overruns of around 200 percent, along with large schedule overruns. Neither contract model makes that uncertainty disappear; they simply decide who absorbs it.

Flexibility

Fixed-price contracts are built around a scope agreed at the start. That works if the scope is right. In practice, requirements often change once people see working software, users give feedback or market conditions shift. Under fixed price, each change becomes a change request with its own price and approval process, which slows things down and can create friction. Under T&M, priorities can be adjusted at the start of each iteration with little administrative overhead.

This is closely related to agile development. The Agile Manifesto, published in 2001, famously valued "customer collaboration over contract negotiation" and "responding to change over following a plan". Agile methods fit naturally with T&M, or with hybrid models, while strict fixed-price contracts tend to push projects towards a more sequential, plan-driven approach.

Quality and incentives

Each model creates different incentives. Under fixed price, the supplier makes more profit by finishing faster, which can encourage them to do the minimum needed to meet the specification and to interpret ambiguous requirements in the cheapest way. Under T&M, the supplier is paid for time, which in theory removes any reason to rush but in poorly managed relationships could reduce urgency. In both cases, a reputable supplier, clear acceptance criteria and good governance matter more than the contract type itself.

Administrative effort

Fixed-price projects require substantial up-front effort to write a detailed specification, plus ongoing effort to manage change requests and acceptance. T&M projects require less up-front documentation but more ongoing involvement from the client: setting priorities, reviewing progress and making decisions regularly. Neither is effort-free for the client.

Budget certainty

The main appeal of fixed price is certainty. Many organisations need to know exactly what a project will cost to secure approval or funding. That certainty is real but only applies to the agreed scope. If the scope turns out to be wrong, the total cost of getting what you actually need, including change requests and follow-on work, can exceed what a well-managed T&M project would have cost.

When fixed price is the right choice

Fixed price is a good fit in a specific set of circumstances, and when those conditions are met it can work very well for both sides.

The scope is small and clearly defined. A brochure website with a known number of pages and agreed designs, a specific integration between two systems with documented APIs, or a well-understood feature added to an existing product are all reasonable candidates. The more precisely the work can be described and the fewer unknowns it contains, the more realistic a fixed price becomes.

Requirements are unlikely to change. If the outcome is determined by external rules, such as a regulatory reporting format or a defined set of compliance changes, there is little reason for scope to shift during the project.

You have done this before. Repeat work, such as a new website built on an established template, or a second app for a different market based on an existing one, can be estimated far more accurately because the team has real data from previous projects.

Budget certainty is essential. Some organisations, particularly in the public sector or businesses with strict capital approval processes, need a firm figure to proceed. In these cases, fixed price may be the only practical option, and the priority becomes defining the scope well enough to make the price meaningful.

The work is a discovery or assessment. Short, defined pieces of work, such as a discovery phase, a technical audit, a design sprint or a security review, are often well suited to fixed pricing because the output is clear even if the eventual build is not. (Internal link suggestion: a guide to the software discovery phase.)

The common thread is low uncertainty. When the cone of uncertainty around a project is narrow, a supplier can price it accurately with a modest risk margin, and the client gets genuine certainty without paying heavily for it.

When time and materials is the right choice

Time and materials tends to fit projects where uncertainty is high or the work is ongoing.

You are building a new product. New products, such as a startup's first app or a new customer platform, rarely end up exactly as first imagined. Feedback from users, testing and stakeholders reshapes priorities, and the ability to change direction quickly is valuable. A detailed fixed specification written before anyone has used the product is likely to be wrong in important ways.

Requirements are complex or unclear. Projects that involve complex business rules, legacy systems with poor documentation or integrations with third parties whose behaviour is uncertain contain unknowns that are hard to price in advance. Under fixed price, these unknowns become either large risk premiums or disputes.

The work is continuous. Ongoing development, maintenance, improvements and support do not have a natural end point. A T&M arrangement or a dedicated team model, with a regular budget and priorities reviewed every few weeks, suits this kind of work far better than a series of small fixed-price contracts.

You want close involvement. If you have a product owner who can work closely with the development team, set priorities and make decisions quickly, T&M lets you direct the work in detail and get the most value from each iteration.

Speed to start matters. Writing a specification detailed enough for a reliable fixed price takes time. T&M can start sooner, with the scope refined as work progresses.

The main condition for success is active management. T&M works well when the client has someone with the time and authority to set priorities, regular visibility of progress and spending, and agreed mechanisms to control the budget. Without these, costs can drift.

Hybrid models that combine the two

In practice, many of the best-run projects do not use a pure version of either model. Several hybrid approaches combine the budget control of fixed price with the flexibility of T&M.

Fixed-price discovery, then time and materials

This is one of the most widely used and sensible combinations. A short discovery phase, priced as a fixed fee, produces a clear understanding of goals, users, scope, technical approach and a prioritised backlog with a realistic estimate. Development then proceeds on a T&M basis, guided by that plan. The client gets a reliable picture before committing to the main spend, and the supplier does not have to price uncertainty it cannot yet see.

Capped time and materials

In a capped or "not to exceed" arrangement, work is billed on T&M terms, but total spending cannot exceed an agreed limit without the client's approval. The cap protects the client's budget, while the team retains flexibility within it. Caps work best when combined with regular reporting, so that both sides can see early if the budget is likely to be reached before the priorities are delivered and can decide what to change.

Fixed price per milestone or iteration

Here, the project is broken into smaller phases, such as an initial release followed by later releases, or a series of short development iterations, each with its own fixed price and scope. Because each phase is small, it can be estimated more accurately, and the scope of later phases can be adjusted based on what has been learned. This gives frequent budget certainty without locking in the whole project at the start.

Dedicated team

For ongoing product development, many businesses contract a dedicated team of agreed size for a monthly fee. It is effectively a form of T&M with predictable monthly costs, where the client sets priorities and the team works continuously on the product. It suits long-term work better than project-based contracts.

Target cost with shared savings and overruns

In a target cost model, the parties agree an estimated cost. If the project comes in under target, the savings are shared; if it runs over, the overrun is shared up to a limit. This approach is more common in large construction and engineering contracts than in typical software projects, but it can align incentives well on larger software programmes where both sides want to control cost without the rigidity of a fixed price.

A decision framework

Choosing a contract model comes down to a few practical questions. Answering them honestly usually points to a clear option.

How well defined is the scope? If you can describe every feature, screen, integration and acceptance criterion today, and those descriptions are unlikely to change, fixed price is realistic. If key decisions are still open, or you expect to learn from users, start with discovery or choose a flexible model.

How likely is change? For a new product, a market that is moving or a process the business is still refining, assume change will happen and choose a model that makes it easy. For regulatory work or a replacement of a well-understood system, change may be limited.

How important is budget certainty? If the organisation cannot proceed without a firm figure, fixed price or fixed price per milestone may be necessary. If you can work within a budget range and manage it actively, capped T&M often delivers better value.

How much time can you give the project? T&M and agile approaches need a client-side product owner who can prioritise and make decisions regularly. If you cannot provide that, a fixed-price project with a clear specification and less day-to-day involvement may be more realistic, although you will still need to review and accept deliverables.

How long is the relationship? One-off, short projects lend themselves to fixed price. Long-term product development suits T&M or a dedicated team.

How much do you trust the supplier? T&M requires confidence that the supplier will work efficiently and report honestly. If you are working with a new supplier, a small fixed-price first engagement is a low-risk way to build that trust before moving to a more flexible arrangement.

Contract clauses that protect you

Whichever model you choose, the contract determines how well you are protected when things do not go to plan. The following points are worth getting right. They are general guidance rather than legal advice, so involve a lawyer for significant contracts.

For fixed-price contracts

A precise scope and clear exclusions. The specification should describe what is included in enough detail to avoid arguments, and list what is excluded. Ambiguity is the most common source of fixed-price disputes.

Acceptance criteria. Define how each deliverable will be tested and what counts as complete, including how long you have to review and what happens if defects are found.

A change control process. Agree how changes are requested, estimated, approved and priced, and how they affect the timeline. Rates for change work should be agreed in advance.

Milestone payments linked to accepted deliverables. Paying against accepted work, rather than dates alone, keeps incentives aligned.

Warranty. A period after acceptance during which the supplier fixes defects at no extra cost is standard and important.

For time and materials contracts

Agreed rates for each role. List hourly or daily rates for each type of team member and any expenses that can be charged.

Budget controls. Include an overall cap or approval thresholds, and require the supplier to warn you early if spending is likely to exceed estimates.

Transparent reporting. Require regular timesheets or reports showing who worked on what, progress against priorities and budget consumed.

Team composition and continuity. Specify the seniority and, where possible, the named individuals on the team, and require notice and handover if key people change.

Flexible termination. The ability to end the engagement with reasonable notice is one of the client's main protections under T&M.

For any contract

Intellectual property ownership. Make sure the contract assigns ownership of the code, designs and documentation to your business on payment. In both the UK and the US, work created by an independent contractor does not automatically belong to the client in many cases, so an explicit written assignment matters.

Access to code and environments. Code should be stored in repositories you own or can access at all times, with infrastructure and accounts in your name where possible.

Data protection and security. If the supplier will handle personal data, include a data processing agreement and security obligations.

Exit and handover. Define what the supplier must provide if the relationship ends, such as code, documentation, credentials and reasonable support during transition.

Managing the project under each model

A well-written contract is only the starting point. How the project is run day to day matters just as much.

Managing a fixed-price project

The client's main tasks are to make sure the specification is right before work begins, to respond quickly to questions, and to review and accept deliverables promptly. Delays on the client side, such as late feedback or missing content, can give the supplier grounds to extend the timeline or charge extra.

Keep a log of change requests and their cost and time impact. If changes start to accumulate, step back and ask whether the original scope was wrong. Sometimes it is better to agree a revised scope or switch the remaining work to a different model than to keep adding change requests to a fixed-price contract.

Test deliverables against the agreed acceptance criteria rather than against new expectations. If what was specified no longer meets your needs, that is a change request, not a defect.

Managing a time and materials project

Under T&M, the client takes a more active role. Appoint a product owner with the authority to set priorities and the time to work with the team, often several hours a week or more. Maintain a prioritised backlog, so the team is always working on the most valuable items.

Review progress and spending regularly, ideally at the end of each iteration, with a demonstration of working software. A burn-up chart, which shows completed work against the total planned scope over time, makes it easy to see whether the budget is likely to cover the priorities and where trade-offs are needed. If the scope keeps growing, the chart shows it clearly.

Track the burn rate, meaning how quickly the budget is being spent, and compare it with what has been delivered. If the two diverge, address it early by reprioritising, simplifying features or adjusting the budget.

Under any model

Regular communication, early visibility of problems and shared ownership of decisions matter more than the contract type. Projects tend to go wrong not because of the pricing model, but because problems are hidden until they become crises.

Common mistakes when choosing a development contract

Choosing fixed price for a project that is still undefined. A fixed price based on a vague brief is either heavily padded or likely to end in disputes. If the scope is not yet clear, invest in discovery first.

Assuming fixed price means no risk. Fixed price moves cost risk to the supplier, but the client still carries the risk of receiving something that meets the specification yet does not meet the real need.

Running T&M without a budget or product owner. Time and materials without active prioritisation and spending controls is how budgets drift. Assign an owner, set a cap and review spending regularly.

Comparing quotes on price alone. A low fixed price may reflect a narrow interpretation of scope, a less experienced team or optimistic assumptions. A low hourly rate may come with slower delivery. Compare what is included, who will do the work and how changes are handled.

Writing a vague specification. Under fixed price, every ambiguity becomes a potential argument. Under T&M, unclear goals lead to wasted effort. Either way, clarity about what you are trying to achieve is essential.

Treating change requests as a sign of failure. Changes are normal in software development. What matters is having a fair, efficient way to handle them and the willingness to revisit the contract model if change becomes the rule rather than the exception.

Locking into one model for the whole relationship. The right model for the first phase may not be right later. Many relationships start with a fixed-price discovery or small project and move to T&M or a dedicated team as trust and understanding grow.

If you are planning a website or software project and are unsure which contract model fits, it is worth discussing the options openly with potential suppliers. A good development partner will explain the trade-offs honestly, suggest a structure that matches your level of certainty and help define the scope well enough for any pricing model to work.