When you put an addition on a house, a foundation is needed in order support the new addition. However, many people choose to pour a crawlspace foundation instead of a full basement foundation because they do not want to add another staircase or think it is too hard to cut through the existing concrete foundation. This could not be further from the truth. Cutting through the foundation is fairly simple. It is just messy.
Cutting through the foundation
First, measure the width of the opening you want and mark it out on the floor. Transfer these measurements to the wall using a level. Take a concrete saw and run the saw along the lines you drew in order to create an outline as the line may disappear once you start cutting. It is important to note that you will be performing the cutting from the unfinished side of the wall if possible.
Put on a pair of eye protective wear and a respirator mask before starting the cutting. Have a friend hold a hose a few inches away from the blade as you cut in order to keep the dust down. This process will take several minutes to cut one line. Make sure that you go all the way to the sill plate that supports the framing. Once you have cut up both sides of the opening, hit the cut slab with a sledgehammer along the edge of where you cut. This will continue the cut seam through the concrete wall. Cut a horizontal line through the center of where the opening will go and hit along the cut with the sledgehammer. At this point, it is all sledgehammer work. Hit along the cut line starting at the top until you start to get pieces coming off of the wall. Eventually, the first half will loosen and be able to be pushed in. It is easier to break the large piece up if you go to the finished side and push the concrete into the addition basement where it can be broken up into smaller pieces.
Once the entire opening has the concrete removed, you can either cut the slabs into smaller pieces to move, or break them up with the sledgehammer. A combination of both usually works the best.
Make sure that you properly support the sill plate where the concrete used to be with a wood header made of a minimum of a double 2×10 supported by wood studs directly underneath, attached to the foundation.