An addition to an existing house (like an additional bedroom) would require the following:
If you follow the checklist you should have everything that you will need. If you have potable water on the property already, you will not have to prove potable water again, however if there is a septic on the property and you are adding another bedroom, a septic review is required. We call septic reviews "O&M"s (Operations and Maintenance inspections); depending on the type of system you have you either do this inspection once a year or once every three years.
For your floor plan we would like to have a before-and-after. Your building plans should only show the new work that is being done.
Lastly, if your property is located within shoreline or critical areas we recommend that you meet with one of our land use planners first to verify that you will not need some type of land use permit. You can walk in and meet with a Planner Monday-Thursday 10:30-12:00; additionally, Planners are available for scheduled appointments Tuesday-Thursday 1:30-4:30. The charge is $50 for every 30 minutes. Book time with a Planner here: https://www.co.jefferson.wa.us/facilities/facility/details/dcdstaffconsultations-3.