Are Buddleia roots big?

As I said, it's a massive stump, but it's not actually a massive root - just three main thickish roots, all of which were simply lopped off once I had removed enough earth to be able to see them clearly.

Do Buddleia roots cause damage?

Buddleia can cause structural damage if it gets a foothold within pavements, chimneys and the smallest cracks and crevices of buildings. Once this happens, the roots of the Buddleia plant have the strength to force buildings apart, creating a gateway for water into the property.