Turning a SFH into a duplex?

[Steve Volk]

I have the opportunity to buy a 3/2 SFH that has a full basement that was once a downstairs living area for a family member. I'm thinking that my best ROI with this property would be converting the basement into a legal separate apartment and renting it out as well.

My question is, has anyone had any experience in doing this? Is it as easy as just registering the new apartment with the post office for an address after it this space has been inspected and passed code as a livable space? Has doing this been a nightmare for anyone? 

I know the answer may depend on where I'm located (Johnstown, PA) but I wanted to throw this out to hopefully get some feedback.

Thanks!