Every project starts out with a plan. Sometimes it's not an elaborate plan and it may be little more than a sudden inspiration. But even that qualifies as a plan in my mind.
Sometimes the plan is absolutely specific in every detail. The location of everything, the profiles of all of the moldings, the sizes, everything is spelled out. And, as a general rule, the author of plans like this tend to think of them as being worthy of placement on the top shelf, right next to the Bible.
But, I have always tried to see the plan as being more of a set of guidelines than a sacred document. Kind of like the Ron Popiel rotisserie oven. That was accompanied by an incessantly repeated statement that you could "set it and forget it.” But if you ordered one, the first thing you noticed were the big yellow warning labels plastered all over the box informing you that "set it and forget it" should "not be taken literally."
What happens, more often than not, is that we come off the line with a great deal of enthusiasm for the plan. Then, as the project progresses, inevitable, unforeseen complications arise and these usually require some modification of, or deviation from the plan. A.K.A. changes.
Of course, if you are building from your own plan and there is no client approval involved, these changes can be made with impunity. In almost every other case, changes require the approval of at least the client and possibly (probably) an army of architects, designers and contractors. And that's not even considering all of the additional persons of influence who may have an opinion of the subject.
The demands of reality can also have a big effect on the plan. Sometimes I find myself saying, "What was I thinking?" After spending hours talking the client into a detail I thought would be great, I end up looking at what it's going to take to make it and then at the budget I have to work with and end up trying to talk them out of what I so enthusiastically talked them into two weeks back.