How to extend an old cavity wall and meet new building regs.

[kerosene]

Hi,
I want to extend my semi at the rear so that the rear wall, which currently steps out by three metres halfway along, is all one wall.
I want to make the entire space into one large room with the rear wall largely an opening for glass folding doors.
However the original wall is 250mm thick (100 brick + 50 cavity batts + 100 aerated blocks).
What I want to know is this.
Is it possible to build the new wall the same as the old one (so that I can then put in a lintel for the glass doors) and make up for insulation regulations by putting celotex or kingspan on the inside face?

k

 

[^woody^]

Only a timber frame will get it down to that thickness

 

[kerosene]

So it's not possible then?
Unless I rebuild the old wall to be the same as the new proposed wall - 100 brick + 100 Drytherm32 batts + 100 Aircrete + plasterboard on dabs.
Trouble is the founds were built for 250mm and it seems ridiculous that I would have to excavate the foundations to increase the width.

What do people do in these situations?
An alternative for me would to simply abut the existing wall but leave 150mm of each wall either side of the joint as a column.

 

[^woody^]

Only the inside leaf would be a timber frame of 100mm - instead of a block wall. The outer skin would still be brickwork, and when its complete, there would be no noticeable difference, and it will sit on that foundation.

 

[noseall]

What do people do in these situations?

Add 50mm to the existing, whether that be insulation or just plain old boxing out.

Simples.