Cost for converting a 2 unit to a 3 unit

[Forrest Williams]

I'm looking at lots of 2 flats in the western suburbs, and was wondering what people's experiences have been with legally converting 2 units to 3. Several of the buildings I've looked at have a basement unit that someone clearly lived in- kitchen, bathroom, etc. However, most only have one exterior door. What is the approximate cost of excavating a second door to bring the place up to fire code and dealing with the village? Has it been worth it for people who have done it? Thanks for any help.