New Foundation Next to Existing Foundat

I’m building a gargage on to an existing garage which is on a two foot block foundation resting on a poured cement footing. The excavation for the new garage will be three feet below the current garage level therefore I will be below the existing foundation and footing. The excavation is only on one side of the garage which includes the 16foot garage door.
What is the best method to reinforce the existing footing after excavation knowing that as I excavate I will most likely pull out or damage the footing that the blocks are resting on? Presently, my plan is to excavate three feet below the existing garage due to grade considerations and install a poured concrete 42 – 48 inch foundation.
Replies
Are your basically just elongating the garage?
No, the existing garage is becoming a room and the new garage addition will be attached with a solid wall between the two.
The current garage is 20x22, the new garage will be 30x40
Ok I was thrown off when you said "The excavation is only on one side of the garage which includes the 16foot garage door" and still am as your footprint is now larger in two directions rather than one "The current garage is 20x22, the new garage will be 30x40"
Regardless, do most of the digging by machine except for the areas closest to the foundation, which you will have to dig by hand, the soil should be quite hard at this depth so leave it beneath the footing, just make the excavation flush to the footing.
Now heres the tricky part, and I'm hoping others will comment as well. Your addition will most likely sink, so you need to either marry the two footing/foundations together, or build them independently.
I'm having a hard time picturing what your addition will look like. If its a light load and the joint will not behave like a lever, You could "marry" the two foundations by drilling and epoxing rebar connections and pouring your new foundation around the existing footing and rebar. Otherwise a movement joint would be needed.
Can you provide more details. Anyone else have suggestions?
Cheers