@Maureen F. and @Jim Adrian - Thanks for the input!
It was my understanding that an ADU is essentially an "in-law" unit but comes with all the elements of full unit (kitchen, bathroom, bedroom, living room, etc.). Check out this tool kit for Mass ADUs.
I think framing the permit request as a ADU versus a brand new built reduces additional paperwork? Not sure yet. Would need some legal/architectural advice here.
In regards to the zoning standards, Boston has one of the most non-sensible/obsolete zoning standards in the country. Zoning laws were put in place in the 1950s I believe and I would say the majority of the existing (100+ year old) houses/buildings are not in compliant (i.e. set backs, FAR, etc.). Almost every new build needs to go through variance or board of appeal.
The new parcel would certainly not be approved (particularly with the parking requirements) for a stand alone built.
The other driver for an attached ADU is to sandwich in the external wall plumbing which has been giving me problem the past few winters here. I can use some tips on that end as well. I've just been advising the tenants to leave the faucet dripping. Just recently had unit 1 redone but unit 2 has a fairly new bathroom and it's cost prohibitive to do any major plumbing /bathroom works at the moment...