Where I live, there can be up to 3 families, or 3 unrelated groups (say 3 college students, for instance), living in 1 single family PRP. It’s still considered single family PRP though, as it was originally built and zoned to house 1 family.
Outer walls of a single family PRP would most likely still fall under being on the property, as that’s something that’s hard to prove at times. It’s also something that staff has addressed in the PRP clarification (outer walls aren’t mentioned, but there are many different terms that can apply).
I’d also like to point out that some graffiti on PRP might not have even been put up by the owners, but a vandal. Graffiti put up without permission is considered vandalism, and typically isn’t eligilbe. This is also layed out in the Unique Art clarification.