2 to 3 family - convert basement unit

[JP Krueger]

Hi world. I have a 2 family in East Boston, MA that had some work in place for a basement apartment when I bought it. The house is currently a 2 family and I was hoping to make it a 3. It might take 35k for the renovation and I would be looking at $1200/month from the unit. Pretty nice ROI. The ceiling height is ideal and the unit would have one egress through the basement door and another through a window. What are the legal steps in this process?