A remodeler's first visit should include a look at the floor structure, the drain location, and the path to the main stack. After demolition, they'll know whether the joists can be lowered or need reinforcing for a recessed pan, and a good contractor will talk through that before pouring a mortar bed.
The plumber sets the drain and valve under a Stoughton plumbing permit, and the gas and plumbing inspector reviews the rough work before walls close. If wiring changes for a fan or recessed light, the wiring inspector checks that too. All of those visits are arranged through the Building Department's office staff at Town Hall.
Before tile, ask for a flood test of the pan: plug the drain, fill the base with water, and let it stand to prove there's no leak. Then the tile, glass, and fixtures go in, followed by final inspections.