Anytime you want to do work in or near wetlands, flood plains, banks, riverfront areas, beaches, and surface water.
Instructions for completing applications are available on the State Website https://www.mass.gov/wetlands-waterways-permitting-reporting. You can also get some guidance from the Engineering Division. However, many applicants find it to be easier and faster to hire an engineer or wetland specialist who are familiar with the process, which can seem overwhelming to people unfamiliar with the regulations. DEP website.