Hi Steve,

I would think of the UFER in the original house as needed in case of lightning or some other unusual (?) situation. The grounding conductor that should be run between the two panels would be mainly to make the 60 amp breaker trip in case of a short in the wiring of the original house, or the feeders leading to it. (I assume that the 60 amp breaker is in the 400 amp panel.)

Many of us (myself included,) could spend a lot more time studying the complicated and interesting subject of grounding and bonding.

Safe Haven Mark