On the house that's unoccupied (4)
I believe the answer is:
free
I believe this is a double definition.
'on the house' is the first definition.
(something on the house is given away free)
'unoccupied' is the second definition.
(similar in meaning)
'that's' acts as a link.
(Other definitions for free that I've seen before include "Without cost; liberate" , "Under no obligation" , "Gratis, for nothing" , "Not working" , "Not fixed; not in use" .)