What is usually under the old floor
In the triple-deckers and prewar colonials around Springfield, pulling up a bathroom floor tends to reveal the same things. Subfloor that has gone soft around the toilet flange. A joist or two notched deeply for old cast iron drain lines. Plaster and lath walls with no blocking behind them for a grab bar or a vanity. Sometimes a floor that is out of level by an inch across the room because the house has settled toward the chimney.
None of that is a reason to panic, but all of it has to be dealt with before a single tile is set. Tile over a bouncy floor cracks. A shower valve set into a wall with no proper framing moves every time someone turns it. The fix is carpentry, and that is the part of the job I care most about getting right.
Framing, subfloor, and blocking
I sister joists that have been cut, lay down new subfloor with the right thickness for tile, and add solid blocking wherever anything will hang on the wall later: vanity, towel bars, grab bars, the shower door, the medicine cabinet. Blocking is cheap when the wall is open and impossible after.
Where the floor is out of level I shim and plane the joists rather than pouring inches of leveler over the problem. A level floor makes the vanity sit right, the door swing right, and the tile grout lines run straight, and that is what people notice without knowing why.
Windows and moisture
A lot of Springfield bathrooms have a window in the shower, which is a wonderful idea in July and a rot problem the rest of the year. I either relocate the window, replace it with a vinyl unit sized to sit above the wet zone, or rebuild the opening with a proper sill pan and waterproofing so the wall behind it stays dry.
Ventilation is the other half. A bathroom fan that vents into the attic instead of through the roof is one of the most common things I find while I am up there on roofing jobs, and it feeds the ice dams and the mold at the same time. Every bathroom I remodel gets a fan ducted through a roof cap or a wall cap with an insulated line.
Working with your plumber and tile setter
I frame, sheath, waterproof, and trim. On the plumbing and electrical I work with licensed trades who I have used for years, and I schedule so the rough-ins happen when the walls are open and the finish work happens after the tile. If you already have a plumber you trust, that is fine with me.
The order matters: framing and subfloor, rough plumbing and electrical, inspection, cement board and waterproofing, tile, then trim, vanity, and fixtures. Trying to shortcut that order is how remodels end up with a shower pan that leaks into the kitchen ceiling below.
Small bathrooms in old houses
Most of the bathrooms I work on are small, because they were added to houses that were built without them. I have opened up closets to gain a foot, pocketed a door to save the swing, and moved a wall six inches to make a proper shower fit. Those are carpentry moves, not plumbing moves, and they are what turn a cramped 1920s bathroom into one that works.