If your client comes to you and wants something extra, then create a change request that details the additional hours required to make it happen, and exactly how that will affect the overall project.

If your client doesn’t really want said changes, they won’t sign the estimate and you can continue with your project.

If they do sign the request, then you have in writing that they will pay the additional costs. There will be no surprises when it comes to bill for your hours worked.

You can only estimate for what you know about.