Business cases and requirements are two different things.
In this Project Management Q & A session, I answer the question “What does a PM do if the Business Case is missing a requirement?”
The first thing to understand is that Business Cases DO NOT include requirements. Requirements are captured in a separate Business Requirements Document (BRD).
On the other hand, a Business Analyst would usually write the Business Requirements.
Business Requirements should be developed with the Business Subject Matter Experts (SME’s). These people have the understanding of the BAU operations. They will also understand what they require for the change to be bought about through the project.