Expect the remodeler to measure carefully and ask about the person's needs now and in a few years. Common pieces include a curbless or very low-curb shower, a fold-down seat, a handheld spray on a slide bar, a comfort-height toilet, lever handles, a wider door, and solid blocking in the walls so grab bars can go wherever they're needed later.
A curbless shower means the floor must slope to a drain without a lip, which may involve lowering the subfloor between joists. In a house with a basement under the bathroom, the remodeler can check that framing from below. Permits for building, plumbing, and electrical work all come through Lakeville's Building Department, filed online through Viewpoint.
If the plan adds a full bathroom rather than reworking an existing one, ask whether the Board of Health should review it, since your septic system will serve the new fixtures.