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.
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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.'