I have five detached garages on a 3 story corner property in Brooklyn.  Our zooming is R5 My question is can I built two more living space units on top of the garages? How would I do this? And how much will it cost? 

You will need to contact your local buildings department office for those answers. No one on here can go beyond speculation.

Thank you Thomas! Do you know of any companies that take care of all the paper work and building plans?