Buddleia bushes near house - root spread?

I just wasn't thinking when I planted these a couple of years ago - I love buddleia bushes and two of them were cuttings (one of them I moved some months ago due to it seemingly having a problem with the growing tips, maybe due to waterlogging because of a dodgy gutter downpipe, but I replaced it with some small ones that I bought).

Ah well, live and learn. I have quite a few buddleia bushes in my garden, all of them far away from the building I hasten to add.

Amazingly enough, even though the most well established one was but a small cutting two years ago, its main stem is now over an inch thick.