A primary bath remodel usually starts with a design visit: the remodeler measures the room, checks the framing under any heavy tub, looks at how drains reach the main stack, and discusses ventilation and lighting. If walls move or the suite expands, expect a building permit and, possibly, drawings.
Plumbing goes through the town's plumbing and gas inspector, and wiring through the electrical inspector, both in the Building Department, with applications filed through its online system. If the project touches the septic system or adds a bedroom, the Board of Health becomes part of the process, which can affect timing.
Larger remodels take longer and involve more trades. Ask for a written schedule with inspection points, and plan for life without the primary bath during the work.