That's a very handy new function, especially for labeling, although I might get confused with the mathematical iff!
I presume the expression for the problem above would be something like:
iif(house_number, house_number, "")
or to conditionally bring in the building name:
iif(building_name<>"",building_name, iif(house_number, house_number, ""))
This is all much easier than string/expression wrangling with left$(), len(), etc.