What is Downstairs ?
Downstairs is (adv) on or to the lower part of a building He heard a noise in the kitchen and went downstairs to see what it was. I left my cup of coffee downstairs. Synonym below (adj) on the ground floor of a building The house has a downstairs bedroom. You can use the downstairs loo. (noun) the ground floor of a building The downstairs has three rooms. The downstairs of the house is larger than the upstairs. Compare upstairs
source: Easier English, Student Dictionary Upper Intermediate Level