Legalities of turning 550sqft detached garage into in-laws apt.

[Jerson D.]

im curious... i have a detached 2 car garage with its own meter sitting right next to my house. the space is 550sqft with a walk up attic, i was thinking of adding another 500sqft in front of my garage and extending the attic to the new space to be built i have enough space.. hows the legal part of this work? in im eastern mass about 30 mins from boston. i want to either have my girls parents live there for cheap rent  as an inlaw's kinda deal or move in myself and rent my unit... is this even possible.