I know you understand this TAB, and so do some of the others, but I state these things from experience for the benefit of those who have not been through it.
When you enter a project like this you always add 20% to the initial, and still plan for some overruns. As the Bible says, do not start the project if you cannot afford it in time, money, or other resources.
Also, there is never a time when it will be cheaper to do the extra or make it right than before the building project, or early into the project. Examples include full basement vs. partial or only crawl space.