Just a note that I couldn’t place any buildings. I adjusted the code to assign the Physics.CheckBox to a bool and debugged the bool. This showed I was always colliding - so could never place the building.
On investigation, I had a BoxCollider on my root Building prefab AND the cube in the UnitSpawnerPreview prefab. Removing the latter fixed the problem.
Turns out, colliders collide with colliders. Who knew?