Suppose you’re on a bus/train. How do you ask the other passengers if a seat is unoccupied?

OR

Is this seat free?

OR

Is this chair/seat empty?

Is this chair/seat vacant?

Now, with the word ‘house’ or ‘apartment’:

The house has been empty for some days now. There is nobody in the house but it does not mean that you can occupy or buy it; it may be that the residents are out of town.

The house has been vacant for some days now. There is nobody living in the house and so if you want to live in it by renting or buying it, you can do so.