Good stuff. This reminded me of a story a builder told me. A new deck was built and so an electrical receptacle was required to serve the deck. This extended the service such that AFCI was required. However, the old panelboard could not accommodate, so a service change was necessary. The service change then triggered the smoke/CO alarm mandate, whereas the deck alone would not.Another problem is when smoke alarms are required to comply to IRC in existing houses. IRC says they are not required when adding a porch or deck. But what if it is an enclosed porch? Would it be really an "addition" and not a porch?
If the porch walls and roof had over 40% glazing then it's a "Sunroom" per IRC definition. Would it still be considered a porch also and smokes in the existing house are not required to comply to IRC?