Middle Grade/YA book about kid moving into apartment building

I read this book in the late 90's, though I think it was from the 70's or early 80's. I don't remember a lot of details except that it's about a boy who befriends a kid who moves into his apartment building. They make ice cream sundaes at some point, and the fact that the building skips the floor 13 is brought up.

I believe the cover was white, purple, and magenta?

If anyone can help me figure out what this book was it would be great!