The new bit is to cover the bit where there is nothing now.
To expand on this, eight bays (if I've counted correctly) of the roof of the Digby-Wyatt extension to Brunel's train shed were demolished in the 1970's to accommodate the current signal box. Personally I think it would be appropriate to restore these bays as they were, because the original roof complemented the main 'express curve' train shed;
this image shows how it was.
gently settle into the swamp on which it stands...
I don't think there is any particular problem with subsidence; I was alluding to the fact that Temple Meads is in a pretty shocking state at the moment and that if current levels of investment continue then it will eventually fall down.