1 Adding new floor space for a simple bedroom and closet with a foundation (but no bathroom) costs about $25-$50 a square foot if you do all or most of the work yourself (requires high-level skills), and anywhere from $50-$150 or more a square foot to hire a builder, depending on complexity and local labor rates. If you're adding weight to your foundation, you may need a report from a structural engineer at a cost of $300-$800 or more, to see if your existing foundation needs to be reinforced or repaired. 2  You'll probably need a set of plans for your bedroom addition. 3  Your builder might be able to supply them.