upstairs

II. upstairs2 noun
the upstairs one or all of the upper floors in a building OPP the downstairs:
• Would you like to see the upstairs?

Oxford Advanced Learner's Dictionary

upstairs

noun singular

the floor or floors in a building that are above the ground floor

• We've converted the upstairs into an office.

Opp: downstairs

Cambridge Advanced Learner's Dictionary

Cambridge Advanced Learner's Dictionary - 4th Edition

upstairs / ʌpˈsteəz / / -ˈsterz / adverb [ before noun ] , adjective

A2 towards or on the highest floor or floors of a building:

an upstairs landing/window

He heard glass breaking and ran upstairs to see what had caused it.

→ Opposite downstairs

upstairs noun [ S ]

Sadly, the upstairs of the house was gutted by fire.

Collins COBUILD Advanced Learner’s English Dictionary

upstairs

/ʌpsteə(r)z/

1.
If you go upstairs in a building, you go up a staircase towards a higher floor.
He went upstairs and changed into fresh clothes...
≠ downstairs
ADV: ADV after v

2.
If something or someone is upstairs in a building, they are on a floor that is higher than the ground floor.
The restaurant is upstairs and consists of a large, open room...
The boys are curled asleep in the small bedroom upstairs.
≠ downstairs
ADV: be ADV, n ADV

3.
An upstairs room or object is situated on a floor of a building that is higher than the ground floor.
Marsani moved into the upstairs apartment.
...an upstairs balcony.
ADJ: ADJ n

4.
The upstairs of a building is the floor or floors that are higher than the ground floor.
Frances invited them to occupy the upstairs of her home.
≠ downstairs
N-SING: the N

Merriam-Webster's Advanced Learner's Dictionary

Merriam-Webster's Advanced Learner's Dictionary:

upstairs

³up·stairs /ˈʌpˈsteɚz/ noun
the upstairs : the upper floors of a building
• We've decided to rent out the upstairs.
• We're having the upstairs carpeted.