Plumbing, fuel gas, and electrical work is regulated at the state level in New Hampshire. Most other contractors (building, HVAC, etc.) are regulated on the city or county level.
The plumbing and fuel gas licenses are issued by the New Hampshire Mechanical Safety Board, which is part of the NH Fire Marshal’s office. As of 2018, applicants are asked to take their required exams prior to submitting their application for the plumbing license.