Adding 있다 is much more specific for something that is currently happening at the moment of discussion. For this example it doesn't make tons of sense as the state of a house's emptiness is not very fluid. But it's the difference between "I eat meat" vs. "I am eating meat," "the house is empty" vs. "the house is empty [right now, ongoing]"
Maybe it is the difference between a house being empty because no one lives there and a house being empty because no one is there currently?
The former is something the house is, a description of it, whereas the latter is something that the house is right now so, something that it is doing... even though it isn't the house's choice.
It sort of makes sense to me but correct me if I am wrong!
My guess is that 비고 있어요 would mean "is being emptied" while 비어 있어요 means "is ['continuously'] empty." This guess is based on the tips and notes for this lesson:
"Stative verbs like 앉다 or 서다 take the continuous differently. Of course, it is possible to say 앉고 있어요, although this would translate more closely to "I am being seated" than "I am sitting."
To truly say "I am sitting" you would say 저는 앉아 있어요.
The rule here s to take the Casual form and add 있어요. In grammar books this may be written as 아/어 있다."
But if a native Korean speaker could confirm or correct, that would be much appreciated.
I am not a native, but apparently 비고 있어요 does not exist, at least in this sense of the word. Is being emptied appears to be 비우고 있어요. Is in the state of having been emptied would then be 비워 있어요. 비우다 is the active form, 비다 is the stative. I am left doubting that 앉다 and 서다 are solely stative verbs as they are not limited to this grammatical pattern.