Adding a third unit to a two-flat??

[Scott M. Fletcher]

Has anyone converted a two-flat to a three-flat?  If so, what is a guess on adding The building I am looking at has a very spacious attic (7 foot peak) and I am considering converting it into a 3rd bedroom or adding a whole other unit (assuming the title and zoning allow it).  The property is in a Chicago suburb.  

Does anyone have a guestimate on what this would cost?

Thank you.