Your question is definitely noteworthy. I imagine whether it is required or not is going to vary.
My opinion is yes, it should be added (as in required to be connected), where there is new electrical work to be done in the building addition. If no new electrical work??? I don't feel the requirement to add it should be based solely on whether the service is being modified.
I look at it this way both installer and inspector should have certain items on a checklist (physical or mental checklist take your pick)
Check list for installing a service should include:
1. service conductors - size, type, overcurrent protection, load calculation, etc., issues related to specific conductors or cables.
2. service equipment - rated for service equipment, 110.26 issues, load calculation, number of disconnects, etc.
3. grounding -grounding electrodes, grounding electrode conductor, system bonding jumper, identification of grounded conductors
check list for installing a branch circuit will not contain same items may look more like:
1. what is the load supplied and which code section(s) apply?
2. what is the maximum load and what size conductors are needed - including deration issues if encountered
3. raceway or cable installation issues.
4. equipment grounding
there may be more to the list but grounding electrodes or grounding electrode conductors is not something that would normally come up in a check list for branch circuits or feeders that do not supply separate structures.
Again it doesn't hurt to add a CEE to the system - and I do use the new footing for that in many cases but not because I feel I have to - I think is just good idea if there is no current CEE.
