I live in an older house (1939) that was built pretty much as an inexpensive bungalow. The house has been added to over the years--a second floor in 1979 and a 550 sf one-story addition two years ago set on piers. The old part of the house
(25x25) stands on cinder block footings that are deep-set, three per side and one in the middle. Under the house is a 4 foot high crawl space with a dirt floor.

I want to dig out a finished basement under the old section (25x25) without raising the house. Unfortunately I have no idea what this will involve and how much this will cost. What I do know is that the current block footings go below the floor level of the would-be basement.

What can I expect in terms of cost, inconvenience, and overall effort?