Why is there a 2 inch step down into my 1931 bathroom? The bathroom floor is 2 inches below the floor just outside the bathroom door. It is the original floor in the bathroom as well as the floor outside the bathroom.
I'll jump in. The 1-inch range is typical, so a bath mat or entry mat wouldn't interfere with the door. They were meant to be water tight rooms, so caulk or re-caulk the bottoms of everything sitting on the floor.
Otherwise, it may be structural settlement from outright strain, most old places weren't built quite right for the literal tons of the bathroom.
One horrible thing that started to become the new trend was Builder's Grade. Due to bathroom floor construction back then, the structure was lowered to accommodate the double thick bathroom floor. But, the new trend was to put in 1/4-inch low-grade finished floor rather than the standard 3/4-inch quality floor for the rest of the house. You probably got switched backed by the builder to 3/4" & the bath therefore "suffered"...consider a shag carpet mat at the door.