If 5,000 square feet represents one-quarter of the second floor storage space, then the second floor has 5,000*4=<<5000*4=20000>>20,000 square feet of storage space.
Since the first floor has twice as much space as the second floor, then the first floor has 20,000*2=<<20000*2=40000>>40,000 square feet of storage space.
Altogether, when empty, the entire building has 20,000+40,000=<<20000+40000=60000>>60,000 square feet of storage space.
Since the second floor has 5,000 square feet of storage space filled with boxes, then there are 60,000-5,000=55,000 square feet of unused storage space still available in the building.
#### 55,000