How much lumber will you need to build a 30' x 30' Square room addition to your home?

To really determine the quantity of lumber, known in the industry as a "takeoff", you need to provide some more info. This would include at a minimum; is it just the four outside walls? Stud,floor/ceiling joist spacing. Rafter spacing as well as roof pitch. Any doors or Windows? How many floors? If multiple floors, then stairs also. There are many more factors involved than just the square footage dimensions. A good set of plans(blueprints) would answer all these questions.