User blog comment:Wildoneshelper/My proposed category tree (revised)/@comment-127769-20140211171100/@comment-3225604-20140212140137

To reply why building is a unit. Let's check the dictionary of a "unit" in Oxford Dictionary:

"An individual thing or person regarded as single and complete but which can also form an individual component of a larger or more complex whole"

The keyword is "thing", which means it does not limit to people only.