A first visit is expected to end in a number. It works better ending in a list: the dimensions now known, the constraints now fixed, and the three or four questions that cannot honestly be answered until something is opened. That list is worth more than a confident figure produced for a room whose subfloor nobody has looked at.
Most of a useful visit is spent outside the bathroom. Where a basement or crawlspace gives access, the ceiling below shows joist direction and depth, the trap arm serving the tub, whether the drainage under the floor is cast iron or plastic, and whether an old stain rings the toilet flange. The attic shows where the existing fan duct terminates — one that stops short under the insulation is common enough in Island houses of a certain age, and it turns a finish job into a ventilation job.
More can be settled on the spot than most homeowners expect. Toilet rough-in is a tape measure from the finished wall to the closet bolt centres — 305mm is standard, 254mm and 356mm both turn up in older houses, and the answer decides which pans fit without moving a drain. Whether the shower wall is an exterior wall, how much ceiling depth sits above, whether a window sill would land inside a new enclosure, whether the door could be rehung to swing out into the hall: all answerable with a tape and a level.
What cannot be settled is the condition of whatever is covered. Whether the tile is on cement board or on drywall, whether the subfloor around the flange has gone soft, how many layers of flooring are stacked under the vinyl, whether a wall carries load. Each is written down as an open question with its likely answer and what changes if the answer is the bad one. Naming them at the start is what makes a later change a priced decision rather than an argument.