When I was at Leeds University the ground floor of the Physics Admin building was Level 6 (it went up to Level 11). It was built on a hill and according to lege

When I was at Leeds University the ground floor of the Physics Admin building was Level 6 (it went up to Level 11). It was built on a hill and according to legend this allowed room for expansion (which never happened) down the hill without requiring negative floor numbers. To add to the fun the only continuous corridor through the entire T-shaped building was on Level 10, one end of which (the base of the T) was at ground level on its part of the hill. There was a famous 'kink' in the middle of the top / cross-bar of the T (apparently the longest corridor in Europe) because they started building from each end and were slightly off.