80s Life visited some of the season one and two opening credits locations in Hollywood. All of the scenes with the cast were filmed in the Los Angeles area. It's pretty cool the basketball court is still there.

https://www.youtube.com/watch?v=rmACFfZs-iI

rmACFfZs-iI

There's also a video on the building at the corner of Park Ave. and 79th St. in Manhattan.

https://www.youtube.com/watch?v=t1-jslSz_xc

t1-jslSz_xc

Reel Estate: A visit to the ‘Diff'rent Strokes’ building on Park Avenue
https://retrologist.com/post/21386006711/reel-estate-a-visit-to-the-diffrent-strokes

Reel to Real Filming Locations
https://rtrlocations.blogspot.com/2015/12/diffrent-strokes-1978-1985.html

This guy is so good. About a year ago I posted his Small Wonder opening location. How in the world does he find this stuff.