TimW formulated the question :
Elderly parents may need to abandon the stairs. We have a good sized
downstairs utility and boiler room so removing some storage and installing a
sink and shower shouldn't be too hard. I am wary of getting specialists in
who will mark everything up x2 at least and thinking of specifying the
requirements and hardware (enclosure, taps etc) and supervising trades
myself.

Have you thought about installing a stair-lift?