Basement Studio Apartment?

[Ryan Cormier]

I am buying a 3 unit house with a basement (4th unit) that was built but not permitted. I am new to bigger pockets and real estate and am hoping someone may be able to give me a bulletpoint step list of how to go about getting this studio apartment legal. The house is in Massachusetts.

Basement has bath with sink, shower, toilet. It has kitchen with range and fridge. It has gas hook up in living room for a gas parlor heater. 

Any advice would be great appreciated. 

P.s. !st and 2nd floor may have knob and tube, does it make sense to pull down walls and redo or have electrician just poke holes patch and paint?