Converting a single family to multi family (duplex)

[Jason Munger]

tl;dr - what do you need to do to LEGALLY begin using a single family home as a multi family rental property?  (yes, I know this may vary by city, but I'm looking for basic advice to get started)

Looking at a property that is currently listed as a single family home.  I've walked the property and it is a mess (that's what I'm looking for!).  The layout was really odd and I dismissed it because the flow of the floor plan was awful.  Then it finally hit me that someone had been using it as some sort of multi family setup.  Based on some of the items left behind, I'm guess a contractor was housing his laborers?  (not judging...just based on the things left behind)

The property is two stories and each level has it's own full kitchen, living room and 3 bedrooms.  The two stories can EASILY be "disconnected" by blocking the staircase and building an exterior staircase to create a private entrance to the second floor.  It's in an "unrestricted" area of the city and I'm looking into the permit requirements to do this.