Block any sidewalk, halls, or stairways, and you will not use them except to go to and from the unit. 51.

The third floor is currently unfinished space, but has a stairway to make it a legitimate floor.

It is zoned SFR but the previous owner has created a nicely done, including a nice kitchen, mother in law suite with it's own access.The suite has a stairway with lockable door separating it from the rest if the house, allowing two parties to live separately.I would like to rent out both spaces separately, but was informed by a good investment realtor I use that renting the property as a duplex could be troublesome since it isn't zoned that way.

The owner shall maintain all means of egress at all times in a safe, operable condition and shall keep all exterior stairways, fire escapes, egress balconies and bridges free of snow and ice, provided, however, in those instances where a dwelling has an independent means of egress, not shared with other occupants, and a written letting agreement so states, the occupant is responsible for maintaining free of snow and ice, the means of egress under his or her exclusive use and control" 

And before the entire spectatorship of his death—all obstructing walls have been torn down and the great figures of the empire stand in a ring upon the broad, soaring exterior stairways—before all these he dispatched the messenger.

Since you've mentioned no financial limitations I would maximize the income potential of the property by building the maximum number of living spaces that are allowable and appropriate (for the neighborhood and property), enlarge the existing house, create a living unit above the existing garage, create a multi-unit building, and build a second garage.   1st, I'd look into maximizing the size of the existing home with an eye toward adding one or two appropriately sized bedrooms each with its own bath (this depends on how the home is laid out);  2nd I'd create another 2 bed 2 bath Air BnB space above the existing three car garage with a separate stairway entrance to each bedroom. 

When I lived in Massachusetts we had apartments with two doors and we still had fire escape stairways where we could exit through a window if there was a fire.

I had exactly  this kind of situation in a NJ house we lived in, and when we moved we added an interior basement stairway but you give up some space when you do that. 

Is the internal stairway which is currently open between the garden and Duplex apartments or internal between the 2nd/3rd fl duplex?

I'm doing research on a contractor I am considering hiring to replace some stairways at my 10-unit apartment building.