PMI's Pulse of the Profession reports that 52-55% of projects experience scope creep. Deloitte's outsourcing research quantifies the cost: scope churn adds 20-40% to outsourced engagement costs. That means a $200,000 outsourced project actually costs $240,000-$280,000 by the time scope changes are priced in.
This is not a failure of planning. It is a feature of software development. You learn things during the build that change what you need to build. The question is whether your engagement model punishes learning or embraces it.
Why Scope Changes
Scope changes because reality is more complex than the spec. Every project starts with assumptions. Some are wrong.
The user research said customers want feature X. After building it, the analytics show nobody uses it. The API documentation said the integration works this way. In production, it works a different way. The database design assumed 10,000 users. The product hit 50,000 in month 3.
