@Runner @mooncube I know I'm reaaaally late lol but!
If it's cc-00 or open you don't have to credit, if it's cc-by you have to, if you bought it or it's in the program you're using (ex. csp) then no worries.
This applies to draw-over/paint-over, but no one can tell you shit if you're using it as a reference.
In this screenshot you can see the 3d model made with floorplanner and a request I've been working on, they're similar but not a drawn-over, which means I don't have to credit.