Rdddk

Hi I am building a timber frame first floor over a block cavity wall. I have the outer block one course higher than the inner and a 100mm cavity, I was hoping to have the floor joist crossing the cavity by 90mm so that i could use 195mm timber plate sat on top of the outer block and the joists so that i can build 195mm timber frame and not loose the space in the rooms the first floor is to be cladded and the ground floor rendered. My building inspector has said my joists cannot hang over the cavity which means i will potentially have to build the wall 300mm thick. Can anyone suggest an alternative option?