How to split duplex even further?

[Halil C.]

Hi everyone, (I was not sure if I should post this in multi-family section or short term rental section, however, to me it sounds like a house hacking)

Got a duplex recently where each unit has 3 bedroom/2 bath. Planning to occupy one of the units and use the other one as STR. However, 3 bedroom is too big for me. The layout is very convenient that I can easily put a wall where there is a door separating 1 bed and 1 bath (master bedroom) from the rest of the unit. This master bedroom has sliding door to outside.

The question is: How do you utilize one unit as 2 units? 

A) Do you apply for efficiency -not sure if there is an application for it-? I don't know if it is called "efficiency" when you have a studio within the unit (without a new construction outside the property) 

B) Or, do you apply to the city for converting the duplex into an official triplex (replacing the door with a dry wall)?

C) Or, do you just keep the door, still keep it as one unit, but basically separate your renters' and your space with a lock?

More context: I need to have a small kitchen in that bedroom so not sure how to get a permit for 2nd kitchen if the option C is the most logical. Also, planning to use the other side of the unit as short term rental.

Any advise which could make this more seamless would be highly appreciated!!