I have seen this before when the local council have paid to put a shower in for someone old (as part of their senior care duty). Relatively easy to quickly bang an electric shower in somewhere like that. Likely far easier/cheaper than revamping a whole bathroom to add a shower.This house is stunning but why is there a shower in one of the bedroomsi think i'd need a little more privacy!
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That or maybe to accomodate a lodger in that room or the B&B explanation already given. There is a sink next to it. Not sure what the lodger does about the toilet (when they need a no. 2 anyway)