A project without Additional Services is pretty rare. No one likes to talk about an increase in fees. Which do you prefer? Losing money, but having a content client? Or getting paid fairly even if there is some discomfort involved?
Or maybe you are working by the hour or your fee is a percent of construction cost. Charging by the hour takes care of additional work automatically. The fee as a percent of construction cost might work if the change increases the cost of construction proportionally to the extra work. Often it doesn't. A fixed fee never gets you paid for a change. So here's what you do. Adopt a simple one-page form that you can complete in a minute or two and email to the client for a yes or no. We use this simple form. Here are the ways that it plays out.
We have found that no one faults you for being business-like unless they were hoping to take advantage of you. Getting paid for everything that you do is a form of Business Development!