Or when does a detached structure become attached? What if there is a masonry patio wall connecting both structures? Or a breezeway? Or a continuous concrete slab? Trying to determine when a supplemental grounding electrode system is required.

I always assumed it was the building envelope. If you gave to go outside to get to another part, it is another building.

Definition of detached structure. detached structure.

1. A section of a building that is not directly attached to the main building.

A garage that is on a property but not directly attached to a house is an example of a detached structure.

That definition is certainly ambiguous. Would a masonry wall attached to both structures not be "directly attached?" A porch or breezeway is certainly directly attached.

There is a block wall extending from my house to the garage in front, open in the back and the tax man, the building department and the real estate people say I have a detached garage (ground rods etc for the electrical inspector)