Are there guidelines that determine when a house should be brought up to code when it's being remodeled?

Here's an (incomplete) example:

I tore the quest out of a 4-unit dwelling, replaced everything with pex/copper. We swapped out the washerbox to a new one with pex valves (instead of leaving the box and just swapping the valves), the inspector said the horizontal run of the washerbox discharge had to be 3"

(There's more to the story of course, but I was curious when a house had to be brought up to code if, for the most part, the drains weren't touched.)