The home owner engages you to look after their property and pets while they are away and you need to be in attendance to be able to fulfill that obligation. In return you are offered free accommodation. Having said that, everything is negotiable with the home owner. For example, if the sitting position is for a reasonably long term and you want a break, the home owner may be able to arrange for a neighbour or friend to come in for a couple of days. You need to discuss this with the home owner before the house sitting assignment commences so that arrangements can be made beforehand as they are unlikely to be in a position to be able to make alternative arrangements while they are away.