1. Decide on the square footage of the room you will be adding to your home. Room additions can cost up to 50 percent higher, between $80 and $130 per square foot before considering wiring and plumbing, than that same room had been constructed with the rest of the house, according to the Home Addition Plus website.. Decide on the square footage of the room you will be adding to your home. Room additions can cost up to 50 percent higher, between $80 and $130 per square foot before considering wiring and plumbing, than that same room had been constructed with the rest of the house, according to the Home Addition Plus website.