Home addition Johnston cost will depend mostly on what addition will you want to add to your home. Its size, the complexity of the design, workers, and its location will all have an impact on the cost of your home addition Johnston project. The size and complexity of your project will determine just how many contractors, electricians, licensed plumbers, carpenters and even decorators you will need to accomplish your project. Add to that the cost of the inspection before the commencement of the project. If your Johnston home addition is a big project, it is advisable to hire a general contractor. Get biddings and estimates from several contractors within your area to know the overall cost of your project. General contractors will take care of managing your project from the beginning to the end. They will also provide the manpower needed and the materials, so that will save you a lot of time hiring different persons and purchasing your construction materials. Additional work like replacement of old pipes, repairing or replacement of floorings will also add to the overall expense.