@Robert Nelson
I agree with everyone that it's a long process. I've been working on my conversion for over 6 months now. Most of the properties in Baltimore city are in the R8 zoning district. The land requirement for that zone is 750sq ft of space per dwelling. (ex. 1500sq feet for two dwelling unit) Most properties in the city do not have that much land and in order to increase the residential density from 1 to 2 units, you would need conditional use approval via a City Council ordinance. This potential ordinance would also have to include several zoning variances, including lot area (density) and off-street parking. However, usually your council representative for that district won't even consider it unless you get approval from the local community.
If you are looking to house hack in Baltimore city you could look for a home with a separate basement entrance and rent up the upstairs without converting it to a multi family. There are few caveats like not being able to separate your gas/electric bill (only one meter allowed), or "technically" you shouldn't have 2 kitchen stoves as this requires a two dwelling unit....
If you are interested in looking at zoning here are some resources:
http://cityview.baltimorecity.gov/webmap/index.html?webmap=d8055e343a1442c7bb61fcfb2f06a183
https://ca.baltimorecity.gov/codes/Art%2032%20-%20Zoning.pdf
Good luck