Quite similar to What does "bis" mean in an address?, I have sometimes seen "ac" or "acc" in a number over a door in the street in Spain. This is taken from Google Maps from a street in Seville:
I don't know if this is a general rule for all Spain, or some cities only, or it may be something outdated as I can't find any modern-looking numbers with that abbreviation, modern-looking numbers just say 42 and 42A.
What does "ac" or "acc" mean in this context?