Home extension

We own a two story home and want to do a potential extension on the second floor where we would add a master suite. For context, the back of our house is slanted and our living room is two stories and the addition would be above our current living room making that just a normal height. We are redoing our roof and siding soon which is what sparked the potential for the addition. Where does one even begin with this process? What do you think costs of something like this would be? The measurements would be about 16 ft wide by 30ft. Our current master is on the main floor adjacent to the living room so essentially we would want the upstairs master to be the same length as the living room and master on the main floor. The plumbing for the bathroom is also there it would just have to go up straight.