Most people have been here at some point or another – in your project, you get really small requests along the way that you’re happy to take care of, but at some point the little things add up. Sometimes it takes less time to implement something than it does to re-negotiate the project plan.
Providing the spec/requirements plan is decent and it isn’t a doomed project to start with, at what point do you actually blow the whistle and start re-negotiating? At any request? When that request requires additional pages / forms? Or just feel it out? Would love to hear how you make the call.
Budget N hours of ad-hoc requests in your project plan. (You know it’s going to happen, so why isn’t it in there?) Then track your ad-hoc requests and renegotiate when the budget’s blown.