In the back of my house I have a 1/2 bath with the washer and dryer in the bathroom. On the other side of the wall there is a 12'X12' slab of concrete. I have thought about using that slab by having a contractor build an additional room there and making it be a utility room and making the bathroom into a full bath. How much would this cost? And/or would it be cheaper to do-it-yourself?