Can I add a legal rental unit in basement (Boston - Dorchester)?

[Maureen F.]

Hi, I'm a newbie to this great community!  Currently my son is buying a three unit house in Dorchester.  The basement has a high ceiling, about 12 feet.  The seller mentioned that at one time there was a unit down there, long before his ownership, although I don't see mention of it in the building permits.  How hard would it be to get approval to develop a unit there?  What are the criteria and process?  I know you need two means of egress.  Thanks for any and all thoughts!