1.
If you go downstairs in a building, you go down a staircase towards the ground floor.
Denise went downstairs and made some tea.
? upstairs
ADV: ADV after v
2.
If something or someone is downstairs in a building, they are on the ground floor or on a lower floor than you.
The telephone was downstairs in the entrance hall...
? upstairs
ADV: be ADV, n ADV
3.
Downstairs means situated on the ground floor of a building or on a lower floor than you are.
She repainted the downstairs rooms and closed off the second floor.
? upstairs
ADJ: ADJ n
4.
The downstairs of a building is its lower floor or floors.
The downstairs of the two little houses had been entirely refashioned.
? upstairs
N-SING: the N