"Which floor is it?"
Translation:Vilken våning är det?
Just for clarification on the two words for "floor": "golvet" refers to the structure (as in, "the apple fell to the floor"), whereas "våning" refers to a level of a building (as in "I'm going to the sixth floor").
Did I understand that correctly?
Normally våning. On its own, plan could mean so many things it's an impractical word. And if you say våningsplan, it's unambiguous, but it gets too long. But plan can be practical on maps etc because it's shorter, so it might be printed more often than it's said.
No, that's usually more like "what kind of floor is that?" rather than "which specific floor is it?" While you can use it in both senses, I don't think it's idiomatic enough (nor prevalent) to accept both.
Since våning is a noun of common gender, don't we need to say Vilken våning är DEN?
The 'det' in this sentence is a so called "impersonal subject" (Sw: "formellt subjekt") and in Swedish this is always 'det'. In English either 'it' or 'there' are used as impersonal subjects (There is... / It is...).