A 10-by-15-foot bedroom addition, for instance, might cost $900 to $1,200 just for framing. An addition that includes a kitchen will require plumbing and boost costs. An addition with a kitchen can run $250 to $300 a square foot, not counting finishing details like appliances and countertops. An addition with a bathroom can cost $125 to $200 a square foot, again with a number of cost variables including type of flooring, shower and tub, plumbing appliances or custom lighting.