By making your existing roof to become a balcony, mean you got to add in concrete to become your balcony floor at lease, that will not recommend to do so, coz adding concrete structure mean will adding load to your existing house beam and column, which the existing column are expected to support 2 storey house only, that will be dangerous to do that, However, if you really intend to do so, you can get an engineer to do the loading calculation for you, if they willing to sign on the document. 2nd thing is local council is very hard to approve your application, specially is obviously see from outside the house