This is frustrating the hell out of me: I’m normally pretty damn good at answering these questions!
So. Family buys a house. It used to belong to a giant. All the rooms have been divided up into more sensible, human-size rooms as that’s easy enough to do, but the bathroom hasn’t: you can’t divide a collosal bath and reconfigure the plumbing.
They use the bath as a pool, and the different soaps add different characteristics to the bathwater. Mysterious minty-green glow, etc…I think.
I can’t remember much about the adventures which ensue, to be honest, though there may have been a person-flushed-down-plughole episode? Maybe? It’s the premise that sticks in my head, not the story.
I’d say I probably read it in the mid 80s, in the UK.
Any ideas?