Adding square footage to your home typically costs $150–$400 per square foot, with bathroom additions on the higher end and garage or sunroom additions on the lower end. Use this free home addition cost calculator to estimate your project based on size, room type, foundation choice, and finish level. Results include a full breakdown across foundation, framing, exterior, interior, and mechanical (HVAC, plumbing, electrical).
Foundation choice and addition type are the biggest variables. A full basement foundation costs 4–5x more than a slab. Bathroom additions cost more per square foot than bedrooms because of plumbing and tile work. Garages are cheaper because they're unfinished. Full-story additions (building up) save on foundation cost but add structural reinforcement to the floor below. Finish level can swing the final number by 30–50%.