situation1. new requirement of existing project; situation2. a new project;
1: Review the additional requirement and inform them how much time/cost it will add to the originally agreed deliverable.
2: Review the requirements and outline how long it will take, define milestones, make it clear what the agreed specs are, establish how much it will cost, etc.
Mike Monteiro has a couple books you should read if you haven't already...
Thanks for your help! :)