A bathroom in a Stratford historic building raises a genuine technical conflict, and it is worth setting out plainly.
A modern bathroom is a sealed, waterproofed box. A timber framed or solid walled historic building works by letting moisture move through the fabric and evaporate. Putting the first inside the second without thought traps water against historic timber, which is the most effective way of rotting a frame that has stood for four hundred years. The answer is not to avoid bathrooms in old buildings, it is to confine the fully tanked wet area to where it is genuinely needed and to keep the external fabric breathable everywhere else.
Ventilation carries more weight here than in a modern house for the same reason, and an extractor discharging into a roof void puts moisture straight into the structure you are trying to protect.
There is a consent dimension as well. In a listed building, subdividing a historic room to form an en suite, removing original joinery or cutting into a decorative ceiling can all need listed building consent. Where a room genuinely cannot take a bathroom without damaging what makes it worth listing, we will say so, even though it is not what anybody wants to hear.
Floors bring the usual older building considerations of long spans and previous notching. Our bathroom installations page sets out what a full refit involves.