How to define software requirements

The process that separates a successful project from one that spirals in cost and timeline

9 min

Most software projects don’t fail because of technology — they fail because of poorly defined requirements. Ambiguous features, uncontrolled scope growth, misaligned expectations between business and development: it all starts in the definition phase.

Defining requirements isn’t about writing a 200-page document nobody will read. It’s an iterative process of discovery, prioritisation and communication that must stay alive throughout the project.

Types of software requirements

Software requirements fall into two main categories. Functional requirements describe what the system must do (register a user, process a payment, generate a report). Non-functional requirements define how it must do it: performance, security, accessibility, availability and scalability.

Both are critical. A system that does everything asked of it but takes 10 seconds to load each page is a failure. Similarly, an ultra-fast system that doesn’t cover essential business flows is useless.

  • Functional: what the system does (features, flows, business rules)
  • Non-functional: how it does it (performance, security, scalability, accessibility)
  • Constraints: technical, legal or business limitations (GDPR, tech stack, integration with existing systems)

User stories: the format that works

User stories are the most effective format for capturing functional requirements. They describe a need from the user’s perspective: "As a [role], I want [action] so that [benefit]". This format forces thinking about who uses the feature and why, not just the what.

A good user story is independent (delivers value on its own), negotiable (details are discussed with the team), valuable (provides user benefit), estimable (the team can estimate effort), small (completable in a sprint) and testable (you can verify it’s met).

  • "As an admin, I want to block suspicious users to protect the platform from fraud"
  • "As a customer, I want to filter products by price and category to find what I’m looking for quickly"
  • "As a manager, I want to export reports as PDF to share them with directors who don’t have system access"

Acceptance criteria: when is it "done"?

Every user story needs clear acceptance criteria defining exactly when it’s considered complete. Without them, discussions about whether something is "finished" become a subjective debate that wastes time and erodes trust between business and development.

Acceptance criteria should be specific, measurable and verifiable. Use the Given-When-Then format to structure them: "Given [context], when [action], then [expected result]".

  • Given I’m a registered user, when I log in with valid credentials, then I access my dashboard in under 2 seconds
  • Given the cart has products, when the user leaves the session, then the cart persists for 72 hours
  • Given a payment error occurs, when the user retries, then a descriptive message is shown without losing form data

Prioritisation: what to build first

Every project has more requirements than time and budget to implement them. Prioritisation determines what gets built first, what’s deferred and what’s discarded. Without explicit prioritisation, the team ends up working on whatever screams loudest, not whatever delivers the most value.

  • MoSCoW: Must have, Should have, Could have, Won’t have — simple and effective for aligning stakeholders
  • RICE: Reach × Impact × Confidence / Effort — quantifies impact for objective comparison
  • Value vs Effort matrix: quick visual for classifying into quick wins, big bets, improvements and discards
  • Story mapping: organises user stories into a user flow to identify the minimum viable MVP

Documentation that stays alive

Requirements documentation is only useful if it’s current. A 200-page specification document created before development and never updated is worse than useless: it creates false confidence. A few updated pages beat many outdated ones.

Use tools that live close to the code: stories in Jira/Linear with acceptance criteria, docs in Notion or Confluence linked to epics, and interactive prototypes in Figma as visual specification. Documentation is a means, not an end.

Common mistakes when defining requirements

After participating in hundreds of projects, the mistakes repeat with surprising frequency. Recognising them is the first step to avoiding them.

  • Defining solutions instead of problems: "I want a red button" vs "I need the user to see the primary action"
  • Not involving real users in definition — only business stakeholders
  • Assuming requirements won’t change: they will, your process must accommodate that
  • Ignoring non-functional requirements until it’s too late (security, performance)
  • Trying to define everything in granular detail before starting: paralysis by analysis

Key Takeaways

  • Projects fail more often from poor requirements than from technical problems
  • Use user stories to capture the what, who and why of each feature
  • Define specific, measurable acceptance criteria to avoid subjective debates
  • Prioritise with methodology (MoSCoW, RICE) and build highest-value items first
  • Keep documentation alive and close to the code, not in static documents

Need help defining your project?

We guide you through the discovery and requirements definition phase so your project starts on the right foundations.