Foundation sizes are 21 feet by 21 feet, 34 feet by 46 feet and 21 feet by 32 feet shallow spread/mat foundation type.
It carries load up to 700 kips from the structures. It is founded on stiff clay below 6 feet from existing ground.
How thick are the foundations? What is the UCS or su values for the foundation soils? What were the original loads? What are the new loads?
Are you concerned about the condition of the foundations? Any indications of damage?
What I'm driving at here, is that if you know how to analyze mat foundations, you just treat the existing foundation as if it was "new", keeping your engineering judgement alert for possible pitfalls.
For unknown values or iffy conditions, use your best guess and stay a little conservative.