Improving more than 50% replacement value of a property in NC

[Clint Harris]

My wife and I are interested in completely remodeling one of our beach duplexes that we use for short term rentals. Specifically, we want to go up another floor, new covered porches, new hardy plank, basically a full remodel. Already met with the structural engineer, and interviewing builders now. Here’s the question, The property was built as an upstairs/downstairs duplex in the 80s, each unit is 3/2, if I increase the value of the property over 50% of the replacement value, am I required to bring everything in the property up to code? Would that mean all framing, etc that has changed since then? We purchased for 370k, put 20-30k into it, currently worth 450k. If we DO, then I’m assuming that would be based on the replacement cost of the structure