Like with previous steps, unless you have extensive construction experience, it is ill-advised to build an ADU by yourself.
Otherwise, you will need to hire a general contractor to oversee your project. Often general contractors will have other specialized subcontractors who work for them. Or, they will have connections to other contractors whom they will recommend that you hire. Specialized contractors include those who work with concrete, framers, siding, drywall, painters, electricians, plumbing, finish carpenters, masons, roofers, landscapers, insulation, etc. General contractors, especially good general contractors, will have their schedules filled up months or even years in advance. Keep it in mind that once you hire a GC, they may not be able to start work on your property for quite some time. In this down time, make sure you have your permitting in order, as permitting often requires long waiting periods (see PERMITTING).
Bear in mind that changes in material costs may dramatically change your project’s financial picture, and, if your project is in Vermont, be sure to account for Vermont’s Commercial and Residential Building Energy Standards. Watch this Zoning Administrator Training put on by TRORC for more information.
Also, have a timeline in mind for construction. Construction projects like this often run longer than estimated and can have significant delays caused by inclement weather, especially in snowy Vermont and New Hampshire. A good option to minimize construction delays is to buy a pre-fabricated ADU. Wheelpad and Backyard ADUs have many pre-fabricated homes and designs for you to peruse.
Lastly, discuss with your contractor whether it will be necessary for you to move off-site for any length of time during the project due to any number of reasons, including connecting the ADU to your wastewater system or electrical system.