The cost of construction varies by area.

In Southern California where we are, this might cost you anywhere from $250 to $350 per square foot for the new 2nd floor construction plus the extra cost of adding structural supports and additional foundation at the first floor of the existing residence to support a second floor.

You should look for several contractors and architects in your area. Interview them and ask them what they think it would cost. If you get 3 or 4 opinions from very experienced professionals in your area, you should end up with a good idea of what it might cost.

first question is will your existing structure support another floor and as for cost get some estimates they are usually free,