From Vision to Structure - Writing Functional Requirements with Method
This article is part of an ongoing series where I share a complete walkthrough of how to create a digital product from scratch. If you're just starting this journey, I highly recommend reading my previous post: The First Steps in Creating a Digital Product to understand the context and follow along smoothly.
A well-documented requirements phase is critical in any software development process. It benefits the client or product owner by ensuring that nothing important will be forgotten, it helps the project manager organize the work more effectively, and it allows developers to focus on implementation without constantly asking questions or making assumptions. Clear documentation helps everyone involved stay aligned and productive.
It’s important to understand that requirement documents are living documents. They’re not meant to be written once and followed rigidly to the end of the project. Instead, they should evolve with the product in an iterative and incremental way. Real user feedback is the only reliable source of truth when deciding which features should be built next. That said, your initial document doesn’t need to cover every possible idea. It can focus on early versions of the product while still outlining future plans to give a broader vision of how everything fits together. The sweet spot is somewhere between minimalism and maximalism.
We’ll start by creating User Stories, based on the documents we produced during the previous step in this series, especially the User Journey, which helps define the main goal of the application, and the Personas, which remind us who we are building the product for. User Stories are a great way to capture functional requirements in an Agile, user-centered format. They are perfect during the ideation phase. A simple example of a User Story might look like this:
As a customer, I want to recover my password using my email, So that I can access my account if I forget it.
Using the format "As [user type], I want [action], So that [objective]" to define the User Story.
And it can be complemented with the Acceptance Criteria:
The system must send a password reset email with a unique link.
The reset link must expire after 30 minutes.
The new password must be at least 8 characters long.
As we write these stories, many questions will emerge about how the system should behave. Some of them can remain open temporarily, but the document will only be considered "ready to move forward" once it fully defines the functionality of the version being specified. No feature should be decided during implementation. Ambiguities at this stage are guaranteed to cause confusion and rework later.
Now, how do we organize all these requirements efficiently? I’ve experimented with several methods and tools over the years. Here's a quick overview of what I’ve learned:
A single monolithic requirements document is great for having everything in one place and makes it easy to search with Ctrl+F. But it quickly becomes too large and hard to navigate. It’s also very difficult to reference specific requirements since they’re all mixed together.
Storing requirements directly in task descriptions speeds up development and offers great traceability. However, starting your planning this way is problematic. Tasks are too bureaucratic to create in the early, fast-changing stages, and editing or removing them can be a pain. Plus, you’re tying your documentation to a proprietary format, which makes exporting or reusing it very difficult.
What has worked best for me recently is creating multiple Markdown documents. Markdown is an open, lightweight format widely used in tech. I use Obsidian to easily navigate between files and link related documents or requirements. These Markdown files can be saved directly in the project’s Git repository, making changes easy to track. Once the User Stories phase is complete, requirements can be referenced by link directly in development tasks.
Next, let’s talk about prioritizing requirements. If you're just getting started, your main goal should be to minimize the scope as much as possible to reduce unnecessary implementation costs. Some features can and should be postponed for future updates. Your MVP’s core goals should be to demonstrate value to users and validate your business model—in other words, prove the product can be profitable before expanding it.
Now, how do you keep the documentation clean and up to date? It’s surprisingly easy for it to become a mess full of outdated, incomplete, or obsolete content. One helpful strategy is to create indexes for your documentation. These add an extra layer of organization by clearly referencing which documents and requirements are relevant to a given scope. Also, I strongly recommend assigning exactly two people to maintain the documentation. Not one, not three. One person tends to write carelessly in a way only they understand, and more than two people can lead to chaos. Ideally, this should be the Product Owner and the Project Manager, or the Product Owner and a Business Analyst assistant, who can review and help keep the quality consistent.
To wrap things up, documenting functional requirements is a crucial step that brings clarity, alignment, and structure to the development process. It prevents misunderstandings, helps the team move faster, and lays the groundwork for a smoother implementation.
In the next article of this series, I’ll dive into the design phase—covering Information Architecture, Wireframes, and Interactive Prototypes—to start shaping how the product will actually look and feel.
If you're building a product or just interested in improving your process, feel free to reach out. I’m always open to exchanging ideas and experiences.
Written by Gil, a fullstack developer with over 15 years of experience and a strong focus on practical software architecture and blockchain technology.