Hi, Is it ok to build a cavity wall with a 200mm cavity? I have a 9" solid party wall, and in order to build right up close to it with a 50-100mm cavity, I would need to underpin the wall. The existing footing is 250 below ground, 250 deep and not deemed suitable for the internal blockwork skin we are proposing. An alternative suggested by BC, is to build a new footing (450 wide, 1m deep) next to the existing and then I can build on the edge of that alongside party wall. But the existing footing protrudes 200mm from party wall so it would leave a 200mm cavity. So starting point is, would a 200mm cavity be acceptable?