It can cost more to build a second-story addition than one on the first floor because a second-story addition typically requires that the home’s existing roof be completely removed and maybe even replaced. Depending on the condition of your existing roof, you might be able to lift it off in one or two sections with a crane and reinstall it on top of the new addition. It’s expensive to rent a crane, but not nearly as expensive as paying for an entirely new roof. Additionally, more support structures may need to be installed to ensure the second story is stable and connected to the first floor. Depending on the project, you may also need to relocate during the build, which can be costly if you decide to stay in a hotel or other rental.