Red is a concrete foundation. Black is rotting wood foundation that needs to be replaced.
Foundation issues suck. House has foundation issues. Most of the foundation will need to be replaced. Part of the house currently has a wood foundation which is rotting. There is an addition which has a concrete foundation. (See image above).
I know that the wood foundation can be replaced with a pier and post foundation. They can stabilize the house, dig and pour piers, then use metal posts to brace the house at the right height.
Is a pier and post foundation the only option? Is there a way to do a concrete foundation?
Edit: The image is a top down look at the perimeter of the house. The red part is a ~600 sqft addition. The black part is the main house.
Is the house occupied? If not, can the existing floor/decking be removed to get direct access to the crawl space and supports? If so, that will make the job so much easier.
While you are at it, have you considered digging out underneath the house and creating a concrete and cinder-block basement? It would cost a bit more but since you are already doing major foundation work, it might be worth it and would dramatically increase the square footage of the house. Adding a basement would allow you the freedom to bring in large excavation machines to do the digging with.