In my town the limit is 5 without a permit, so also ensure that 4 is legal in your area before anything else.

I always look at the zoning trends in an area and what permits have been pulled by neighbors ... 

Last year we renovated the basement and turned it into a permitted 450-square foot studio and have been renting it out to traveling nurses every since. 

I have no access to previous owner and permits don't show any roof work or anything for decades, so can't determine its age.There are no leaks, no missing shingles.

Properties where development has stalled may be in violation of stormwater permit requirements.  

I have never added a unit, but you also have to think about zoning, permit fees, architectural drawings, etc. 

Unless you are a pro at eyeballing the cost of the rehab (or have a friend/long term relationship with someone who does this for a living), pay a contractor to go to those 3-5 you identified to get a better eyeball estimate (and make sure your contractor is adding permitting and other fees).

I write a permitting approval contingency in every offer to ensure the strategy can be executed after purchase. 

If you can like Josef Hardi said get in touch with the city, particularly the permitting department they should be able to point you in the right direction for any code related issues.You could also talk to a local electrician, and plumber for concerns related to those trades.

The delays in remapping the flood zones by FEMA, in combination with a business partner fallout, as well as having several GCs and architects incorrectly submit permits and plans for approval resulted in a VERY drawn-out project.