Firstly, thank you all for your responses.
The drawings you see were provided by an architect on A3 size paper and the hand drawn diagram is when I asked for further detail.
The foundation has already been laid at 450mm where the stonework is going -
The idea of the stone addition was for cosmetic appearance purposes - although judging by some of the comments maybe I should increase the insulation and do away with the stonework and perhaps just render? Although don’t really want to go through planning again...
Unforunately I cannot get a drawing from the architect to show the build up above the proposed rsj.Either way it looks as though the general consensus amongst the responses is that I should increase the insulation to i. e 100mm ?
Do you think I could make the wall the same thickness as the rest of the build and either just render or use a stone slip for cosmetic purposes ? If I do it this way then will the 50mm insulation on his sketch be adequate enough ?
At least this should solve the problem with the initial rsj problem to 450mm thick wall.
Any further thoughts or comments would be appreciated. Has been a bit of a nightmare dealing with drawings showing minimal detail.
Thank you all again