When Harry gets his first place after Hogwarts that actually has more than one floor, he comes home after getting a load of boxes to find Hermione using a sledgehammer on the drywall beneath the stairs. And Ron’s like, “Look, mate, I borrowed this stuff from my dad, I’ve got a DRILL and a - what’s it called again, Hermione?” “A stud finder.” “Right, one of those, and we’re going to fix your stairs.” Harry’s like, “But there’s nothing wrong with them.” “Yes, Harry, there is.” Harry’s just sort of standing there in total bewilderment while Hermione totally demolishes the wall. “We couldn’t have done that with magic?” “No, Harry, this is personal. You two take this mess out to the skip.” And then Harry stands around a while longer and Hermione puts in support beams in the appropriate places so the stairs don’t fall in, and Ron’s very excited about using the stud finder even though Hermione won’t let him use the drill. When they’re finished, Harry has this set of shelves. So he says, still completely confused, “I thought we picked this place because it had loads of storage.” And Hermione says, “Go get some of my books. I know it’s just shelves, but it’s not a bloody cupboard.”

And every time Harry moves for the rest of his life, Ron and Hermione are there on moving day and they knock out anything under the stairs, even if it’s just a wall. Hermione reads a lot of books. Ron learns to use a miter saw and a carpenter’s square and practices the nail hammering spell until he can do it perfectly on the first try. And sometimes it isn’t very practical but it looks nice…

And sometimes, when they all get older and have children, it’s cozy and has a purpose…

And eventually Hermione gets the trick of there being nothing under the stairs at all

Which is the story of how Harry Potter never lived in a house with a cupboard under the stairs again for the entire rest of his life.