Ellen G. Friedman, Keller Williams Realty, 617-448-1542 or firstname.lastname@example.org
I didn't know that Somerville new(er) 3- families needed a sprinkler system. That would probably be a $15k- $18k cost for you right there. Definitely check on that....
Also, the layout of the building itself with have a lot to do with how and how much it will cost to convert to a 3 family. You have to deal with lining up the plumbing/ drainage lines, and also you need to have 2 means of egress for each unit. Many times that can mean a back stairway, or an emergency spiral stairwell, etc.
What is the electrical system like currently? That could be a pandora's box if you are adding a unit.
If you'd like any help on figuring this out, let me know.
All a local issue. All depends on what the local authorities say you can or can not do.