@emi - good clarification. ( I noticed your post in chat and came over here. I tutored English to German speakers (Hamburg & Berlin) for many years. )
@Tim - the 'continuous' form is often challenging to translate. You are right in this case to use the simple:
Are you going downstairs/upstairs?
The continuous form (*) is used for things happening in the moment, placed within the verb tense time frame:
I am running.
I was running.
I have been running (all day, so I'm really tired). :)
Here's an instance where it might happen using your example question.
Lets say there's a fire and a lot of smoke in that building. The firemen have been in there a little bit, but can't see each other well. One fireman sees the other on the stairs, but doesn't know which way the other has gone. He might radio:
Are you coming from upstairs ?
Asking indirectly "have you been up there ? Do we need to search there ? " Or "which way do we go next ? " But he'd probably just ask:
Anything up there ?
Here's a trick to remembering this: think of running water.
Statement of an ultimate fact using present tense: Water runs. (It's the nature of water :)
Statement of a condition in the passing moment: The water is running (now). (Oops, you forgot to turn the water off :))
(* in German it's something like "verlaufsform" ? - Laufen = run. But German, like many languages, does not have a continuous form. )