Many cities and counties require a permit for any wall that is over a
certian height. Sometimes it's 3 ft and some areas it's 4 ft..
If your house is in and area that requires a permit, then I would check with
the goverment agency that issues permits.. There should be approved drawings
or details on file..
I will have to admit that I do a lot of things without permits if I feel
confident in my skills and judgement (not an excuse). It sounds like your
wall effects two homes so it may have been built by a licensed contractor or
the developer that built the homes. Here again, the drawings and permits
should be on file..
However!! If you check with the permit office and find that the wall never
had a permit, you run the risk of being required to prove that the wall is
to code requirements, etc..
My opinion and experience. FWIW