What is Upstairs ?
Upstairs is adv on or to the up- per part of something, e.g. a building or bus She ran upstairs with the letter. I left my glasses upstairs. Let’s go upstairs onto the top deck – you can see London much better. adj on the upper floors of a building We have an upstairs kitchen. We let the one of the upstairs offices to an accountant. noun the upper floors of a building The upstairs of the house needs decorating.
source: Easier English, Student Dictionary Upper Intermediate Level