Adding on to a basement

We are going to add on to our kitchen and family room. We would also like to have more space in our basement. We have a 1/2 basement (finished) and would like to expand this underneath the new addition on the main level. When adding an addition, would it be a significant additional expense to dig out the basement for expansion? Or would this need to be done anyway to pour a foundation for the main floor addition and it would only be a matter of digging and pouring deeper.

Thanks