If you are currently working on a system specification and you have covered the functional requirements but not yet touched the non functional requirements, this article is for you. Non functional requirements (NFRs) define the qualities and operational conditions a system must meet to be effective, not just operational. They cover performance, security, reliability, usability, interoperability, and data migration. Functional requirements tell you what a system does. Non functional requirements tell you how well it does it, under what conditions, and within what constraints. Two systems can share identical functionality and still differ enormously in practice because their NFRs sit at opposite ends of the quality spectrum.
In my experience, NFRs are the requirements that get deferred, abbreviated, or skipped entirely during early project phases, then cause the most pain at go-live. I have seen performance thresholds missed because nobody agreed them upfront, and security vulnerabilities surface during penetration testing because the access control requirements were never formally specified. Getting these right at the start is not overhead. It is the work.
The Six Core Categories You Need to Cover
Most NFR frameworks organise requirements into the same core categories. Understanding what belongs in each one helps you ask the right questions during elicitation and write requirements that are actually testable.
| Category | What It Covers | Compliance Relevance |
|---|---|---|
| Performance | Response times, throughput, scalability under load | SLA obligations, service continuity requirements |
| Reliability and Recoverability | Uptime, disaster recovery, batch processing continuity | Regulatory mandates for data availability and business continuity |
| Security | Access control, data integrity, encryption, audit trails | GDPR, sector-specific data protection laws |
| Usability | Ease of use, consistency, accessibility standards | Equality Act 2010 (UK), accessibility regulations |
| Interoperability | Platform compatibility, integration with legacy and third-party systems | Data sharing agreements, integration standards |
| Data Migration | Accuracy, completeness, cutover timelines, contingency plans | Data integrity obligations, continuity of record-keeping |
Performance
Performance requirements define what good looks like under real operating conditions, not just in a demonstration environment. You need to specify:
- Transaction response times: Define the expected response time for key actions such as login, data retrieval, search, and page loading so these can be tested explicitly.
- Scalability thresholds: Specify how the system should behave as user numbers grow, including geographical distribution and concurrent session volumes.
- Peak load periods: Identify business hours, seasonal spikes, and anticipated daily variation so infrastructure can be sized correctly from the start.
Performance requirements matter most when they are tied to service level agreements. If your organisation operates under an SLA with downtime penalties, the performance NFRs are not optional quality criteria. They are contractual obligations.
Reliability and Recoverability
A system that goes down at month-end payroll processing or end-of-quarter reporting is not a minor inconvenience. It is a business failure. Reliability NFRs cover:
- System availability targets: State the required uptime percentage and the maximum tolerable downtime window, including the impact if that window is exceeded.
- Backlog handling: Define what happens to transactions queued during an outage and how the system resumes normal operations once it is restored.
- End-of-cycle processing: Specify requirements for batch jobs and critical processing cycles at daily, weekly, and monthly intervals so these are protected from outage windows.
Security
Security NFRs are non-negotiable in regulated environments and increasingly in every environment. They need to cover three areas at minimum:
- Access control: Define how access to sensitive data and system functions is restricted based on user role, and specify how access is provisioned, reviewed, and revoked.
- Data integrity: Specify controls that prevent unauthorised changes, accidental deletion, or tampering with records.
- Regulatory compliance: Document the specific data protection obligations the system must satisfy, including GDPR where applicable, and translate those obligations into testable requirements.
If you want a more detailed treatment of how NFRs connect to requirements documentation, the article on non-functional requirements templates covers how to structure these in a format your technical team can work from.
Usability and Interoperability
Usability requirements define how intuitive and accessible the system is for the people who will actually use it day to day. Accessibility is not an afterthought. In the UK, compliance with the Equality Act 2010 means digital services must meet accessibility standards, and failure to specify this upfront pushes the remediation cost into delivery or post-launch.
Interoperability requirements address how the system operates across devices, platforms, and integration points. In most enterprise environments I have worked in, there is at least one legacy system that the new solution must connect to, and the assumptions made about that integration during early specification are almost always underspecified. Define device compatibility, infrastructure constraints, and data integration needs explicitly.
Data Migration
Data migration NFRs are frequently underestimated. They need to address accuracy and completeness of migrated data, the timeline and approach for cutover, and the contingency plan if migration fails partway through. For organisations handling sensitive data at volume, an incomplete or corrupted migration is not just a technical problem. It is a compliance breach.
A Real Example: Where This Gets Difficult
On a data platform implementation for a public sector client (Organisation A), I was working through the NFR set during the requirements phase. We had agreed performance thresholds with the project sponsor and the solutions architect: a maximum three-second response time for standard queries and 99.5% availability during core business hours. That seemed straightforward.
Then the infrastructure team came back and told us the platform would be hosted on a shared government cloud environment with resource limits we had not been told about at the outset. The solutions architect said the availability target was achievable. The infrastructure team said it was not, not without a dedicated resource allocation that was not in the budget.
The sponsor pushed back on the idea of revising the SLA downward. From their perspective, the availability requirement was tied to a regulatory obligation, not a preference. We ended up in a three-way conversation between the sponsor, IT, and the procurement team that ran for two weeks. The resolution was a phased approach: the 99.5% target applied only during a defined peak window, with a lower threshold outside those hours, and a formal exception process was documented for outages that exceeded the threshold.
The friction was real and the delay was real. But it was far better to surface that conflict during requirements than to discover it during user acceptance testing or, worse, after go-live. That is the point of specifying NFRs rigorously: not to generate paperwork, but to make these conversations happen at the right time.
How to Elicit Non Functional Requirements Effectively
NFR elicitation works best when you treat it as a structured conversation across multiple stakeholder groups, not a single workshop question. In practice, different stakeholders hold different parts of the picture:
- Business subject matter experts: Can tell you what good looks like from an operational perspective, including the impact of outages, slow response times, or inaccessible systems on day-to-day work.
- IT and cybersecurity specialists: Hold the knowledge on what is technically feasible, what security controls already exist, and where the gaps are relative to compliance obligations.
- Solutions architects: Understand the integration landscape and can identify interoperability constraints that business stakeholders may not be aware of.
Using a structured NFR checklist during your elicitation sessions ensures you do not miss a category. It also gives you a basis for confirming coverage with your project team before you sign off the requirements set.
When you are thinking about how NFRs sit alongside functional and business requirements, the article on business vs functional vs non-functional requirements is worth reading if you need to explain the distinctions to stakeholders or a project team that is new to structured requirements work.
Architectural Principles That Shape NFRs
NFRs do not exist in isolation. In many organisations, they are shaped by architectural principles that constrain or guide solution design. Three principles that come up consistently in regulated environments are:
- Avoid unnecessary customisation: Customisation increases complexity, raises the cost of future upgrades, and can introduce compatibility issues that affect performance and reliability NFRs.
- Adopt established standards: Using industry standards for integration, security, and data formats reduces risk and makes compliance easier to demonstrate.
- Embed compliance requirements in design: Regional data protection obligations, including GDPR, should be treated as design constraints from day one rather than retrofitted during testing or post-launch review.
Getting NFRs right is not about producing a long document. It is about surfacing the constraints, obligations, and quality thresholds that will determine whether your solution actually works in the environment it is being built for. The organisations that treat NFRs as an optional appendix are the ones that end up revisiting scope after go-live, absorbing remediation costs, or managing compliance incidents that were entirely avoidable. Specify them early, test them explicitly, and make sure the people who need to sign off on them understand exactly what they are agreeing to.
Frequently asked questions
What are non functional requirements?
Non functional requirements define the quality attributes and operational conditions a system must meet, such as performance, security, reliability, usability, and scalability. They describe how well a system performs rather than what it does. Unlike functional requirements, NFRs apply across the whole system rather than to individual features.
What is the difference between functional and non functional requirements?
Functional requirements describe what a system must do, such as allowing a user to log in or generate a report. Non functional requirements describe how the system must perform those functions, such as responding within two seconds or maintaining 99.5% uptime. Both are essential and neither can be treated as optional.
What are examples of non functional requirements?
Common examples include maximum page load times, system availability targets, data encryption standards, role-based access control rules, accessibility compliance, and disaster recovery timelines. A requirement that a system must recover from failure within four hours is a non functional requirement. So is a requirement that all personal data must be encrypted at rest.
Why do non functional requirements matter for compliance?
In regulated sectors such as finance, healthcare, and public services, many compliance obligations translate directly into non functional requirements around data security, availability, and auditability. Failing to define these requirements clearly can result in a system that passes functional testing but fails a regulatory audit. NFRs make compliance obligations testable and traceable.
When should non functional requirements be defined in a project?
NFRs should be defined during the requirements phase alongside functional requirements, not after the solution design is underway. Discovering a performance or security constraint after architecture decisions have been made is far more expensive to resolve than addressing it upfront. Early definition also ensures NFRs are reflected in solution assessment and vendor selection criteria.
Try Ash, Your Virtual BA
If you are working through a set of non functional requirements right now and you are not confident you have covered all the categories, Ash can help you close the gaps. Ash is built for BA work, which means it understands the difference between a usability requirement and a functional one, knows what compliance-driven NFRs typically look like in regulated environments, and can help you structure your requirements in a format your technical and business stakeholders can both work from. It is the logical next step once you know what needs to go into an NFR set. Try Ash Virtual BA.
Further reading
- Beyond Functionality: The Critical Role of Non-Functional Requirements | Exclusive Articles for IIBA Members
- 10.30 Non-Functional Requirements Analysis
Written by Sam Cordes, founder of the Business Analyst’s Toolkit.