A successful project must stay in balance and the communication of expectations is critical.
If a project falls out of balance, one or both parties are stuck with a bitter taste in their mouth. Successful projects are those that are delivered on time, on budget and within the required scope. If any of these are missing, someone loses.
A successful project must maintain harmony between project scope, timeline, and cost.
The moment we adjust one side without the other two, we are likely headed for project failure.
One side of the project can only be made longer if one or two of the other sides are also made longer, thus keeping the three sides of the project connected. As you can see if we simply increase the cost of the project to offset our increasing scope, and we are back to where we need to be.
As long as there is an understanding that we are working on a sliding scale of connected parts (a triangle), there's no reason to suggest we cannot get everything we want out of the project.
Communication of expectations is an often overlooked part of a successful project and how important it really is may not be obvious until it goes missing.
Whether it be milestones, additions to scope, project process or procedure, or payment expectations, both parties must effectively communicate their expectations. Think about the last time you really upset. It was probably because what actually happened was not in line with your expectations.
Failed communication often start with assumptions. We may assume you understand that adding to scope is going to result in increased costs, but you may have assumed there is some wiggle room on scope. Both of us may have valid points, but unless we get it on the table, we are at risk later on.