A useful home addition budget depends on the type and size of the addition, the existing house, structural work, foundation and roof connections, building systems, finishes, and work in adjoining rooms. A single statewide average cannot describe all of those scopes. Compare similarly scoped published FHC project examples, then develop a project-specific budget through design and site evaluation. For building upward, see our second-story cost guide. Ask which design, approvals, temporary living, and site restoration costs are included or separate.