Most room additions in Orange County, CA can cost around $125.00 to $200.00 per sq. ft. There are many factors that can change this such as if you are building a second story addition and the cost is $125. sq. ft., then you have to also factor in the amount of structural retrofitting that is needed to stack that home addition on your existing house. That cost might be another $30. sq. ft. making your new total $155. sq. ft. Then adding in the city fees and inspection fees you will start to get a real total.