What happened to Harrys grandparents?
From World Book Day Chat, 2004: They’re all dead and not particularly important to the story, although you will find out a little bit more. From joint interview with Mugglenet/Leaky Cauldron, July 2005: This takes us into more mundane territory. As a writer, it was more interesting, plot-wise, if Harry was completely alone. So I rather ruthlessly disposed of his entire family apart from Aunt Petunia. I mean, James and Lily are massively important to the plot, of course, but the grandparents? No. And, because I do like my backstory: Petunia and Lily’s parents, normal Muggle death. James’s parents were elderly, were getting on a little when he was born, which explains the only child, very pampered, had-him-late-in-life-so-he’s-an-extra-treasure, as often happens, I think. They were old in wizarding terms, and they died. They succumbed to a wizarding illness. That’s as far as it goes. There’s nothing serious or sinister about those deaths. I just needed them out of the way so I killed the
