My subject may be a bit misleading. But here's my question:
What size of a project determines whether or not you have to bring the whole house up to a given code?
There are no nationwide applicable answers to that question. You have to check with your local authorities. For most projects in our area there are two relevant dollar amounts (based on project cost): The first dollar amount is the one triggers the requirement to bring the "entire home to current code". While it is commonly phrased this way, it doesn't really mean absolutely every aspect of the home construction and focuses primarily on safety features. The second dollar amount is perhaps even more important since that is the cap on how much the code compliance upgrades may increase your project costs. A remodel/renovation is not supposed to put you into bankruptcy because you are catching up with 50 years of code improvements. Everything that is newly installed or modified as part of your project must comply to current codes but once you reach the cap you do not have to perform any other code compliance upgrades to other parts of the home. -- Got some spare cpu cycles ? Join Team Helix or Team Starfire!
It may depend on whether the improvements bring to light a safety hazard. We ran into that when we changed the name on the electric account in my wife's old house. The electric department saw something they didn't like and called the building inspector who found "a safety hazard" in the electric coming into the house. Even though the line had been like that for over 40 years they required us to run a new line and put in a new panel for over $2,000. Legal, who knows, but they threatened fines and to shut off the electric until we complied. On a plus side, they didn't make us rewire the whole house.