Here is a situation all too familiar to any contractor: a client calls you on a Friday afternoon and says that his wife wants to move the walkway five feet to the left. Not a big deal, right? You say, alright, no problem, and go on with your (hopefully fun) Friday night plans. On Monday your crew moves the walkway to the left and you are happy that you were able to accommodate your client's wishes at no extra costs. And then the wife comes out and says she wanted to move it to the right, and her husband did not understand her. You are still trying to be a good sport, and re-install the walkway. At this point, you are not inclined to do any favors, and you want to charge the homeowner for extra costs. Can you?
Yes, if you made sure every change to your original contract (please do have an original contract) was documented in writing. It is so easy to do, it is astonishing to me that so many people do not do it. Send an e-mail back to the homeowner, clarifying what was said on the phone (e-mail is better than text, it easier to save it and to find it). All the changes to your contract should be in writing (e-mail is a writing!). Even better if the other party to your conversation confirms the changes back. If they refuse to confirm, ask yourself why, and proceed with caution.
However well your contract is written (hopefully with the help of your trusted outside general counsel), life happens and changes happen. Make sure your changes are in writing, and make sure your contract is allowing for the changes to be made. Ask for the confirmation of changes in writing to avoid future confrontation.