Scope creep in contracting jobs

Yesterday we met with the contractor and the plumber who wanted to survey the job before beginning sometime in the next century.

After answering some of his questions and poking about we asked them some 'wouldn't it be nice' questions, and this is where the problems begin.

Controlling costs of a kitchen upgrade is a problem. If you hear or think any of the following run the other way:
a) It's only a small change
b) Since you're doing this anyway
c) I didn't think about that
d) What's another few dollars
e) Hey that's a good idea
f) Can you estimate this

All of these are trouble and should be avoided to control the costs. That being said, the first two items have already hit the bricks - The bay window and an external gas line.

Under the "can you estimate this" category is the bay window.I hate this thing, it leaks air all winter and since I have 2 other windows (really good windows - only a year old) that we are not going to use I thought put one of these there.

My wife will have none of it, she wants a bay window. So, right now we're estimating the cost of fixing the existing one.

The other item is admittedly mine, the external gas line. This one falls under the "since you're doing this anyway" category. All the gas pipes are moving and the deck is right off the kitchen, so logically why not have an external gas line for the BBQ put in since the whole wall will be open anyway.

We will be getting cost estimates on both.