The most common reason software projects exceed budget and timeline is not technical complexity — it is the absence of a detailed requirements document before development begins.
When clients say "we'll figure out the details as we go," what follows is an endless cycle of change requests, scope creep and rework. Each change after development starts costs 5-10× more than the same change made during planning.
A proper project specification should cover: user roles and permissions, every screen's data inputs and outputs, third-party integrations, edge cases, and acceptance criteria. This document becomes the single source of truth for both the client and the development team.