My individual higher costs may not be applicable to others. The existing electrical panel in my old house didn't have any free space to add on. That sub panel alone was about $2,500. The concrete slab also doubled in cost because we had to raise the floor up 12 inches to get the proper drainage slope of the sewer pipe to tie into the house's plumbing system. Tack on another $2,500. When I was researching pre-fab structures I learned about my local zoning restrictions - no more than 144 sq. ft. for a work/storage shed. That ruled out all prefabs since I realized I had to build up to achieve my 200-square-foot plan. Fortunately, the county rules on height limitations were more generous.  Amazing Modern Backyard Office by Aaron Britt