No matter how well you planned, your home improvement job will change as it moves along. You may encounter unforeseen structural issues, or you may decide to include additional work.
Any good contractor can handle these changes—just make sure all costs are specified in writing first. Tell the contractor at the outset (and put in the contract) that you want to sign off on written change orders for anything that’s going to add to the bottom line of the job. That means the contractor has to give you a bid (a description of the change and a fixed price for what it will cost) and you both have to sign it before the work is done.
This eliminates misunderstandings about the scope and price of the new work. Also, it helps you keep track of your bottom line.