Thanks. There is another good question, as `addr:housenumber=` is the number used with street or village. It's unclear what blocks should use. I have used `ref=` and `addr:unit=` together for now. 
(According to some definition `addr:unit=` can be used when "inside divides into different units or staircases, where certain apartments/flats/offices can only be reached through a specific unit", so it seems more applicable to subbuildings found in modern apartment towers, ie Tower 1A and 1B inside the same Tower 1 physically. Consequently, it looks usable for houses, as different buildings in a compound can be called "units" overseas.  Shop "unit" in a building may be better off using `addr:door=`.)  