My house

I live in a 1904 end terrace which was built as a detached originally, in the 1930 they added 4 terrace houses to the end . It has two basements 12 1/2 feet by 13 feet, 2 same size rooms on the ground floor same area on the first floor that incorporates 3 bedrooms and one bathroom and two bedrooms in the loft . I am considering converting the basements into habitable rooms but have a 7 feet ceiling height to the bottom of the foundations that are of stone . I would like to dig down further to to leave me a 8 feet ceiling height . I assume  i would have to put a ledge or curb in before i dig down any further , the width to be same as the depth i need to dig down .