Construction could take between 4 to 8 months from issue of construction permit to finish for a average sized home. A larger house, with more options custom features, fancier finishes, and more owner involvement — especially if the owner has little construction experience — may take longer. The actual schedule of completion for your specific project will vary based upon many factors including, but not limited to, size and degree of difficulty of the structure, site and weather conditions, material and contractor availability, crew sizes, effective scheduling and communication, the knowledge and experience of those managing the project, and perhaps most significantly, the number, timeliness, and complexity of the changes you will inevitably make to the original contract after construction has already begun.
Large additions, depending on the size and scope, may take about the same time. A smaller addition like adding a bedroom will probably take less. Detached garages may take only 1 month.