Hello Kitan,

According to the plan as described by you it is simple and you have to construct one wall of 4.5 “thickness on RCC slab under the top floor. If you consider plan B for making bathroom and two rooms, you have to construct two numbers 4.5” thick walls on the slab. For making bathroom you have to make necessary arrangements for water supply, pipe line from sewer pipe line. For fixing water closet, washbasin and pipe lines necessary recess should be made into RCC slab and walls. For making bathroom all pipes should be taken from ground level or should be jointed with pipe lines already fitted into bathroom at ground floor. For this purpose necessary dismantling should also done at ground floor. We are in view that if you have no requirement of additional bathroom at top floor level, the plan A is better.
        
If you would supply complete plan of top floor, we were in better position to provide detailed advice about the construction of rooms in the space available at top floor. In this case we have to consider the position of the stair case, opening of doors and entry point of other rooms. Provision of bathroom already done at first floor should be accessed also whether it is sufficient or not.

Thanks