Fixed price or time and materials: choosing the engagement model for your product

The choice behind the choice

Fixed price or time and materials is really a question about risk: who carries the cost when the scope turns out to be different from what everyone assumed. Under a fixed price, the development company carries it. Under time and materials, you do. Neither model is better in general. Each is better for a particular kind of work, and most products go through both.

What fixed price is good at

A fixed-price project has an agreed scope, an agreed price and an agreed date. You know the number before work starts, you can put it in front of your board or your investors, and the incentive to finish is on the company's side.

It works when the scope can be written down well enough that both sides mean the same thing by it. A first version with one core flow. A scoped feature. A prototype. A migration with a known start and end. In those cases the certainty is real, and it is worth having.

What you should know is how that certainty is priced. A company quoting a fixed price is estimating work it has not done yet, so it adds a margin for the unknowns. Agencies that write about their own pricing put that premium anywhere between a quarter and a half of the underlying estimate; Kanopy and Notch both describe the trade-off plainly. You are paying for the number to be a number.

The other cost is rigidity. Every change to the scope becomes a change order, with a conversation and a price attached. On a product that is still being figured out, that conversation happens every week.

What time and materials is good at

Under time and materials you pay for the effort spent, usually monthly, and the scope can change whenever you learn something. It suits work that is open-ended by nature: a product past its first version, a backlog that is reprioritized as users arrive, a team that stays with the product for months.

Its strength is that you never pay for a change order and never build the wrong thing because it was in the contract. Its risk is that spending is only as disciplined as the people managing it. Time and materials without a shared board, a budget ceiling and regular demos is how projects drift.

The discipline that makes it work is visibility: you see what was done, what it cost and what comes next, every sprint, and you can stop or redirect at any point. Ask for that before you sign.

How to choose: three questions

How well is the scope known? If you can describe the first version in a page and both sides agree on it, fixed price is available to you. If the honest answer is "we will know more after the first users", it is not, and a fixed price would only be a guess with a margin on top.

How likely is it to change? A first version of a new product changes by definition, which argues for building it small enough that a fixed scope holds for a few weeks, then switching. A mature product with a roadmap changes by the month, which argues for time and materials with a ceiling.

Who should carry the risk? If a budget overrun would end the company, pay the premium and fix the price. If you can absorb variance and want to keep control of the scope, carry it yourself and pay less.

The combination that usually wins

Most of the products we work on follow the same path. The first version is a fixed-price project: a scope small enough to write down, a price agreed before work starts, a launch date. Once the product is live and the work becomes releases, one after another, it moves to time and materials with a monthly budget, or to a retainer when it needs steady care rather than a full team.

The fixed price protects you when you know the least. Time and materials protects the product when it starts to change. Switching between them is a normal part of the relationship, not a renegotiation.

What to write into either contract

Whichever model you choose, four things belong in writing. The scope, in enough detail that a change is recognizable as one. The change process: how a new request is estimated, approved and paid for. The rhythm: how often you see working software and how spending is reported. And the exit: who owns the code, the designs and the accounts, and what a handover looks like.

How WeaveLines engages

We offer both models, and we say which one fits before anything starts. A scoped prototype, MVP or feature is a fixed-price project. Ongoing product development runs on time and materials billed monthly. After launch, a retainer covers support and steady improvement, and a fractional CTO or CPO takes the seat when a product needs a lead more than it needs more hands.

If you are unsure which applies to you, that is the first thing we work out together, and the answer is usually clear within a thirty-minute conversation.