I'm sure that's a huge weight off your shoulders. 

Either way, I would first look at the amount of available space in your rear yard (measure the lot width, which is probably 50' or so, and the distance from the rear property line to your house, which is hopefully >35') and see if you even have the space to build a 1 or 2 story unit that has your desired bed/bath count. 

- Shoulders slump, eyes drop, he starts to stammer a little at this point, "Uh, well, there might be something I in the process of taking care of".Me - "Great, you get that taken care of and then we will get started on the application, also we don't take cash you will need to convert that to a cashier's check or money order".Never heard back from him.  

If they don't have duplex, look for a single family that is half the width and do mirror images or find a single family that you can turn into a duplex, but still looks like a single family.