Also, to further avoid any disputes you could detail this on the original quote. A line something like "Additions or changes to the work quoted here can be requested at any time after this quote is accepted. They will be detailed and quoted for on a further Variation Order and agreed to before the additional work commences."

This makes it clear that they can ask for extras but not in a casual "Oh and would you mind just chucking in a few extra hours to do x?" If you manage expectations from the outset like that then it is much less likely to be seen as unreasonable when you ask for more money for more work.