How do you answer / respond to clients who commonly demand the addition of the new feature / gizmo / gadget as part of phase I (in this case, as part of a new w

How do you answer / respond to clients who commonly demand the addition of the new feature / gizmo / gadget as part of phase I (in this case, as part of a new website, they ask for a feature not originally scoped but desired to launch with the new site)?