How much should we do? Should we do both business analysis and requirements specification? Questions like these, are present all the time in software projects. The answer really depends on what you would like to achieve see Top-10 reasons to start with business process analysis before specifying requirements.
A fast track method for determining the need for details is illustrated in the figure below. The method focuses on the accept criteria in your project.
If you need the final delivery to only be accepted, if it fulfills specific functionality details, you must specify the detailed requirements – if not, you might be able to settle with the high-level requirements. The method is a great way to initiate the discussion about what details are needed.