When doing some DIY in our loft, I discovered a tall and very flimsy piece of plywood attached to the wall, about two feet above the floor. I pulled some of it away from the wall to reveal... next door's loft! Their loft is completely empty with no proper floor, so I expect they didn't know about this. It's a rented house next door.
Is this normal for houses of a certain age, if somewhat "Magician's Nephew"? It's a 1930's semi.
I'm also wondering what to do about it, thinking of security. The only other access to the loft is through a trapdoor. I suppose bolts on the outside of the trapdoor would be a good start, and some solid wood, or even bricks to board up the gap in the wall!