It is the project manager’s responsibility to manage and control scope creep.
Scope creep is the adding of work to a project without a corresponding update to cost, schedule, and/or quality. Uncontrolled scope creep may render the original project plans unachievable. Scope management is of the utmost importance to the success of the project.
Your best weapon against scope creep is a strong Change Management process.
Once the project scope is documented and approved in the Scope Statement, Project Charter, or project Statement of Work, insist on using Change Orders to document and log all requested changes to the approved project scope.
Evaluate all Change Orders for impacts to the project in terms of cost, staffing, quality impacts, and schedule.
Then get the Change Order approved by the customer/client or project sponsors before implementing the requested changes.
Don’t forget to re-baseline affected the project budgets and schedules after the requested change is approved.