2 Flat addition upward

[John Stewart]

What is the process for building UP on a chicago 2 flat? The building is a 1925 - built two flat that I am entertaining the idea of adding living space upwards. This would not be an additional unit, rather it would just add additional living space to the top floor unit.  I would plan the addition to be "recessed" from the face of the building so the street appeal of the building would not be altered. Does anyone have any knowledge of zoning /city laws that would prevent me from doing this or would permitting process be relatively easy with a good contractor? Thank you