I have accidentally deleted two level 6 buildings in the past week, neither of which was the one I had clicked on when choosing to demolish (I wanted to demolish a level 2 mine in each case). Rebuilding them, not a huge amount of resources, although it'll add up eventually if it keeps happening, but now I have to wait 15 days. Perhaps the concept of what is done cannot be undone would negate an undo button and it would probably get annoying for every building, but at least if I'm demolishing something above a certain level (say 4), a 30 second cancellation timer would be welcome. Noticing within 2 seconds isn't enough because it's already on its way down.