An expression read in American books is "going potty".....as in, "the dog went outside to go potty".

No, it didn't. It went outside to urinate. It didn't sit on its little potty like a toddler being toilet trained. Likewise when the term is used about an adult.

A library where I worked in the mid-late 1990s was upstairs, above a staff room (in a school). We had our own little cubicle, loo, dunny, whatever you call it, tucked away in a corner. The library staff referred to it as "the corner office"......"Is Pat here?" "In the corner office" would be the reply.

It's a term I still use.......