In short, a design and build company consists of all stages of the construction process, from design to build, in one place. A design-build firm works under one contract with the project owner to complete both design and build steps of the project. They will take on the responsibility of planning the project, drawing up the designs, costing, sourcing the building materials and supplies, building the structure, installing any permanent features and possibly advising on the interior design of your home; all while consulting you every step of the way.