I'd imagine it is an outdoor toilet, either a separate building from the main house, or one that can only be accessed from outside. We had one in my childhod home, as well as an indoor one. I've no idea what the builder thougth he was doing, adding one at the ground floor back, when the kitched it stole space from could have used one being accessible from inside.
Thanks for you attempts at providing an explanation. I ought to have given you the context before, but I thought it was such a transparent expression for a native speaker that a context wasn't necessary. Unfortunalety, I was wrong. I say unfortunately because if the term had denotated a clear object it would have been easier for me to understand it and eventually to find a corresponding term in my language. Anyway, here is the context:
"Bungalows features spacious and elegant courtyard bathrooms complete with a tropical garden shower"