The terraced housing in the boot and shoe streets was not built with bathrooms in it, and that single fact explains most of what we find when we open one up.
A bathroom in one of these houses arrived later, usually somewhere between the 1930s and the 1970s, and it almost always went into the rear addition or over it. That addition was built as a scullery or a workspace on the cheapest foundations on the plot, and nobody who built it in 1890 imagined it would one day carry a filled bath, a tiled floor and a tanked shower enclosure. Sometimes it was strengthened when the bathroom went in. Frequently it was not.
So the first thing we do here is look at what the bathroom is standing on rather than what it looks like. A floor that flexes when you walk across it, a door that has started binding, or tiles cracking along a consistent line are all worth understanding before a new suite goes in on top. Fitting a heavier bathroom onto a structure that was already marginal is how a modest refit turns into a structural job halfway through.
Where the structure does need attention we would rather find it at the survey and price it openly than discover it once the old suite is out and you have no bathroom. That is the point in a job when nobody has any good options.
The rest is the usual discipline. Waste runs work to the permitted zones in the joists, extraction goes outside rather than into a roof void, and the wet area is properly tanked because tiles and grout have never been a waterproof layer. Our bathroom installations page sets out what a full refit involves.