How many buildings are in the US?

How would you guys solve this market sizing q? I would first start with residential vs. commercial vs. industrial. However, I'm really not too sure if this is the best approach and would love any thoughts anyone may potentially have. I think my logic is especially shaky for industrial - thanks guys!

Residential:
- Assume 100M households in US (300M people / 3 ppl per household on average)
- Assume, on average, 1 house / apt per household (richer people will have more, some may be homeless, average is probably 1)
- Result: 100M buildings

Commerical (corporations, malls, churches, mom-pop stores, etc)
- Assume 300M ppl in US
- Assume at any given time, ~80% of the US population (240M) is in a commercial building due to work, leisure, etc
- Assume one person takes up, on average, 200 sq ft in each building (since they need room to move around and such, tell me if my logic here is off) so we need enough buildings to fit 48B sq ft
- Assume average building size is 5K sq ft
- Result: ~10M buildings (48B/5K)

Industrial (factories, warehouses, etc.)
- Would likely assume same number of buildings as commercial, as there are probably less people in these buildings on any given day, but they probably have 5-10x the amount of space available to them given safety constraints
- Result: ~10M buildings

Overall: ~120M buildings in the US

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