Sometimes developers (and others for that matter) will use the phrase scope creep, by this what we mean is that what is being requested has changed significantly from what has been agreed.
Lets say we have agreed to build a 5 page website, and the client has agreed to pay £xxx. If when the client sends over their content they send 20 pages instead of the agreed 5 pages. Something has to give, either the amount of content needs to be reduced, or it’s going to take longer to build the site and as a result the price goes up.
When this happens we’ll send the client a revised quote based on the additional work requested, and at that point the client can decide what they want to do.
Some scope creep is inevitable, projects often evolve organically and as such new ideas and feature will often emerge, it’s important however to ensure this is done in a controlled and manageable way. If not, projects can run away with themselves, increasing costs, and missing deadlines.