Yosef Karo

“ If the pile is big and the dedicated space was only a tefach [-tall] carve-out for seven [by seven] tefachs and they carve it out to make a big sukkah, it is not all deemed valid by virtue of the seven-long [careveout] that was made initially. If they dig out four amot from each side, beyond the initial seven, even the initial seven is invalid, as invalid schach invalidates from the side when it is four amot wide. Rem"a: One may not put on the schach before finishing the walls. ”