Perhaps the OP meant to say "You are downstairs". It's not clear that it's a question, after all.
Well, for questions (it has a question mark, after all) which do not start with "czy" we can accept the affirmative word order. Like in: What?! You are downstairs?!
And it is accepted already.
Ah. I was referring to the spoken-in-Polish version which has no rising intonation and no "czy".
Why it is not :na dół ? How to know when it is locative after na or accusative ?
'na dół' (Accusative) is used when your sentence is about movement: "Idę na dół" (I am going downstairs).
'na dole' (Locative) is used when your sentence is about location: "Jestem na dole" (I am downstairs).
My polish friends seen to use a different word for downstairs, something like "parter" or "barter", I'm not sure on the correct spelling or pronunciation but could this be a dialect thing?