I think this says it all........."OH MY"!!
What would you do? Start tearing things apart. Knock down the house. Start from scratch. Talk to those who have done this before. Who do you get involved, because everyone has an opinion, right?
Research is the best place to begin and there are lots of established resources like the city, public archives, community precedent, and professionals. The trick is to get good information, which is scary in today's time as there are lots of people who claim to be experts and/or have expert information.
Leg work and intuition are key to knowing what is the right thing to do. My intuition told me to listen to the land, the property itself, and those who had done trusted work which would require time and site visits. This would involve my family and friends who would come along for the ride and be my trusted sounding board for discoveries.
Step one find out more about the house and the property.