Re: residencial service
If this new addition is incorporated into the older section and has several means to exit and occupy it, than a single service and loadcenter would be okay. If the new section is connected but not accessible to the old section without going outside and then going back in, a separate means of disconnect must be provide to that occupancy. In either case, the service should be sized to handle total connected load. I would than place two, 200-amp main discos at the service location and feed separate loadcenters in each occuppancy location.