I. down‧stairs¹ S2 /ˌdaʊnˈsteəz◂ $ -ˈsterz/ adverb
towards or on a lower floor of a building, especially a house Antonym : upstairs:
Rosie ran downstairs to answer the door.
Charles was downstairs in the kitchen.
—downstairs adjective:
a downstairs bathroom

[TahlilGaran] Dictionary of Contemporary English ▲

II. down·stairs² noun
the downstairs the rooms on the ground floor in a house:
We have still got to paint the downstairs.

[TahlilGaran] Dictionary of Contemporary English ▲

downstairs
adverb
BAD: I ran to downstairs and picked up the telephone.
GOOD: I ran downstairs and picked up the telephone.
BAD: The dining room and kitchen are in the downstairs.
GOOD: The dining room and kitchen are downstairs.

Usage Note:
Downstairs and upstairs are usually adverbs and are not used with to/in/at etc: 'The bathroom is upstairs.' 'I ran upstairs to see what all the noise was about.'

[TahlilGaran] Dictionary of Common Errors ▲