" Does anyone have any knowledge about how much ledge we may end up removing and how much the estimated cost would be? "

Not the slightest bit able to answer this question without an onsite visit and a couple of exploratory digs. Could be an inch.......could be six feet or more. It's anyone's guess without some detective work on your part.

Would also help to know your geographic location as that can sometimes indicate underlying rock formations common to that area. It also has a significant bearing on costs for removal. As well as how appropriate only a 12 inch deep foundation might be.

Thank you for the inputs! We are going to consult a structural engineer to figure out. We are located around Greenwich Ct grrrneich

FYI: "The minimum frost depth for Greenwich, Connecticut, is 42 inches (3.5 feet), as required by the state building code. All foundations and permanent supports for structures must extend at least this far below the finished grade to prevent damage from frost heave."

Yea we plan to do the wall foot at least 42 inches but the other areas we may not need to that deep tho

Not sure you can get away with anything less than the 42" deep foundations. All permanent structures - house extensions - require by code at least that depth.

Hopefully the structural engineer will advise you correctly on this aspect as well.

Thank you for the information! We will confirm. Yeah not sure how much ledge we need get rid of…