I have a building in Brooklyn, Ny which is fully attached on both sides. I would like to build an extension in the rear which will require a below frost line foundation ( 6ft deep). The only access to the rear of the building is through the basement hatch or through the side stairwell. I'm curious if there are hand held excavating machines, cost and time to pulloff a project like this. The area is 25'X20'. Dirt removal is not an issue since there is room from a previous foundation ( behind the proposed extension area) to dump and fill. any advice??