I went to the town of Brookhaven offices and asked questions about the very same issue . I was told heating and plumbing make a SHED not a shed .... but make it an apartment conversion unless its grandfathered in by a CO PROVEN before 1964 ?!! Ok we DO have that thank God ( amazingly thank GOD multiple "vintage "COS were discovered and disclosed before our recent closing by our awesome title company ....)
What we DO have is a derelict 2 car ( but waterfront and with AMAZING inspiring sea and sky views ) garage ( i'm planning to convert it to a painting studio / guesthouse upstairs and a sculpture studio downstairs ) with full plumbing connections to our 1930 vintage cistern /cesspool - with several existing connections to toilets and sinks and showers ( we have multiple dockspace which we hope will help us pay our HORRIBLE taxes !! ) as well as connections to ( long ago ) heat , its own hot water heater etc .
Brookhaven was rather snooty with me regarding my right to restore my own property frankly .
Here's a tip I learned from the title company - they had a fire of their records on the mid sixties - so if U can prove a CO before then Ure golden . I also DONT need to pay any extra taxes as I( or rather the property owners before me ) have been paying extra taxes all along as IF the live-above garage was fully functional ( and BOY does the tax bill reflect that ! ) .