The most common cause of custom software projects running over time and over budget is not bad engineering. It is scope that was not defined before the project started.
The way this plays out is consistent: the client knows what the software should do. The engineering team builds what was described. Six months later, the client says "this is not what we asked for" — and they are both right. What was said was not what was meant, and nobody caught the gap until after six months of work.
Software requirements live in people's heads as a complete mental model that includes every assumption they have about how the system works. When those requirements are communicated, the assumptions do not always come with them. The gap between the spoken requirement and the mental model is where scope disputes live.
The fix is not writing longer specifications — it is writing more specific ones that force the unstated assumptions to become explicit. A requirement is not scoped until a developer can read it and know what "done" looks like without asking any questions.
MoSCoW (Must Have, Should Have, Could Have, Won't Have) is a useful framework, but its value is often misapplied. Teams use it to categorize features and then treat everything in the "Must Have" column as equally critical, and the "Won't Have" column as optional rather than explicitly excluded.
The "Won't Have" list is as important as the "Must Have" list. Writing down what you are explicitly not building prevents scope creep. When a stakeholder asks "can we add X?" in week eight, the answer is not "that will take two weeks" — the answer is "X is in the Won't Have list, here is the change order process."
User stories are good at describing behavior from the user's perspective. They are not good at capturing constraints, integrations, and non-functional requirements. "As a user, I can log in" is a valid user story. It says nothing about whether login uses SSO with Google Workspace, works on mobile, has a session timeout, handles failed login attempts, or meets your company's security audit requirements.
User stories should be accompanied by acceptance criteria that capture everything the user story leaves implicit. Without that, the developer delivers exactly what the story says and not what you needed.
Before writing the first line of code: can you walk through the scope in a 30-minute meeting and have everyone in the room agree on what "done" looks like for each piece? If you cannot, the scope is not ready. Wireframes or low-fidelity prototypes at this stage are the highest-ROI investment you can make in the project — they surface scope misalignments before they become engineering rework.
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