Water towers do not make for easy conversions; there are going to be lots of stairs. London developer Leigh Osborne took a tower dating back to 1867 and converted it into "one of the most lavish and eccentric residences in the city". It was a challenge; he tells the Penarth Times that "There were 2,000 dead pigeons in there, and poo that went up to the top of your wellies!"
An elevator shaft has been stuck on the side, providing access to six stacked bedrooms and the living room in the tank at roof level, 100 feet up.
A large kitchen/dining and living room is added to the bottom.
It's too bad about the elevator shaft; it destroys the proportions of the tower. Perhaps it might have been better if he had eliminated the gym on the ground floor and instead got his exercise running up and down the stairs, like Tom Dixon did.
Lots more photos at First Option, which apparently rents the tower out as a location.