First of all.. Get the disclosures, understand how this unwarranted attic was converted. Most sellers do the work without permit because they don't want to pay more tax and the hassle of dealing with the city. If the work was done by a license contractor and it shows it's up to code, now it's your decision to accept it and buy it as a rental.
Now if the conversion was done by a non-license handyman, then it might not be a good investment to buy.
Also if you are investing, I recommend for you not to buy a condo or townhouse that has rules and regulation in occupancy. Meaning, in a condo or townhouse development they have restriction on the number of owner occupied and non-owner occupied. So, my advice, get a single family house or duplex, tri-plex, etc