Non-Functional Requirements: The Checklist Founders Skip (and Pay For Later)

When I review a founder's spec, the feature list is usually detailed. Then I ask a few simple questions. How many people will use this on launch day? What happens if it is down for an hour? Whose data are we storing, and for how long? Most of the time, nobody has written those answers down. Those answers are the non-functional requirements, and they shape the architecture, the timeline and the cost as much as any feature does.

What non-functional requirements are

Functional requirements describe what the product does: sign up, book a slot, pay, get a receipt. You usually capture them as user stories in your PRD.

Non-functional requirements describe how the whole system behaves while doing those things. The international software quality model, ISO/IEC 25010, groups these qualities into areas such as performance efficiency, reliability, security, usability, maintainability and portability. You do not need to read the standard, but it is a useful reminder of what to cover.

Why they change your quote

Two development companies can read the same feature list and imagine very different products. One pictures a few hundred users and a single server. The other pictures a national launch, strict data rules and round-the-clock support. Both quotes can be fair, and they will still be far apart.

• Scale decides the architecture, caching and how much load testing is needed.
• Availability decides whether you need redundancy across servers or regions, and on-call monitoring.
• Security and compliance add reviews, audit logs, encryption and sometimes outside testing.
• Device and browser support multiplies design and testing effort.

This is why I ask founders to add these targets before they send an RFP. If you want to see how these choices move a budget, our project cost guide and the app cost calculator show the main drivers.

The checklist

Copy this table into your spec and fill in the last column. If you do not know an answer, write a rough estimate and mark it as one. A guess is far more useful to a development team than a blank.

How to make each one measurable

A good non-functional requirement has three parts: the quality, the number, and the conditions under which the number must hold. Here is how I rewrite common vague ones.

• "It should be secure" becomes: staff accounts use role-based access, admin logins need a second factor, and the app passes a review against the OWASP Application Security Verification Standard at the level we agree on.
• "It should handle lots of users" becomes: the checkout flow handles the expected peak load with acceptable response times, proven by a load test before launch.
• "It should be accessible" becomes: customer screens meet WCAG level AA, checked with automated tools and a manual screen reader pass.
• "It should always be up" becomes: a stated uptime target, a maintenance window and a plan for what users see when something fails.

Be honest about the targets. Asking for the highest level everywhere makes the product slower and more expensive to build. A booking app for one city does not need the same availability plan as a payments platform. Pick high targets only where the business truly needs them.

When to set them

1. Before you ask for quotes. Even rough targets make proposals comparable.
2. During discovery. The development team checks each target, flags the expensive ones and suggests cheaper alternatives.
3. Before each milestone. Review the targets that matter for that release, such as load testing before a marketing push.
4. After launch. Real usage data will show which targets were too high or too low. Update the spec.

For mobile apps, security is the area founders most often underestimate. Our mobile app security checklist goes deeper on what to ask for.

Common mistakes

• Leaving them out entirely. The team then makes its own assumptions, and you discover them after launch.
• Words instead of numbers. "Fast", "secure" and "scalable" cannot be tested, so they cannot be accepted or rejected.
• Gold-plating. Demanding enterprise-level targets for a first version burns budget before you have users.
• No owner. Someone has to own monitoring, backups and security updates after launch. Write down who.
• No test plan. Every target needs a matching test, or it will quietly be skipped under deadline pressure.

How we handle them at Creuto

At Creuto, we cover non-functional requirements in the first call and write them into the proposal, which you get within 24 hours of the call. The targets go into the PRD next to the features, each one with a test. We set up monitoring and backups as part of the build, your code lives in your own accounts from day one, and after launch there is a 60-day warranty while real users put those targets to the test.

If you already have a feature list, add the checklist above and send both to any team you are talking to. You will get clearer answers, and you will find out quickly which teams have thought about the questions.