Is it permissible to install an 8’ ground rod inside a building? I see nothing stating otherwise.
There was a rule **I think** about transformers somewhere in 250 that said the electrode had to be outdoors?? It was there, but I can't find it.
Beyond that, AFAIK, there are no NEC restrictions about a GE being located indoors. Guys will say it must outside right at the drip line, so it will be in good, usually damp ground but that is a design issue/opinion. There may be a problem with 250.53(A)(1)- but I don't buy that either for many buildings, esp if there is a basement.
Others have argued that in the event of a lightning hit its better for the rod to be outdoors.....apparently they aren't aware of the ufer.....or that copper water line that juts up thru a slab, both of which are also bonded to the service and will take the same hit-indoors.