Frankenstein Building Reuse

I am currently working on a project that has had 7 different additions dating back to 1956. We are planning on reusing all of the existing floor, foundations, steel framing, roof deck and a large portion of the exterior envelope, so I feel this credit is very likely. However, do to the nature of the building's past, finding accurate drawings of existing conditions has proven difficult. Is there a way to simply state that we anticipate on reusing all of the existing foundation and below grade construction without giving an actual number? Or perhaps I can state that all foundation walls go 36" below grade or greater and use 36" as my average foundation wall kept in my surface area calculations? Has anyone dealt with a similar issue and had success with submission?