It needs to be specified clearly during the interview, or rather in the ad.
The following - dishwasher loading/clearing out, sticking a load of washing in the washing machine and hanging it out to dry every day, cleaning the kitchen after you (and the kids) eat and HELPING the kids tidy up their toys (as opposed to doing it for them) at the end of the day - are NOT considered either housekeeping or cleaning. It's what every grownup does to prevent the house from turning into a dumpster.
The au pair needs to clean her own living space however.
I used to be an au pair many years ago and I was offered to do the cleaning too for extra pay which was fine with me.
In my opinion an au pair isn't high enough on the food chain (of child care) to issue statements like "I don't do housekeeping". If it's a nanny with either a degree from Norland and/or another one in child psychology and/or 15-20 years experience sure, she can get away with it easily.
Au pairs should be willing to do anything that's clearly stated in the ad they replied to. If they aren't, they shouldn't have replied to it.
Hope that helps! Good luck!