I recently completed a 6m by 5m extension to an end of terrace house, I put in 100mm celotex in the old and new sections floors, front room of the old house with 50mm pir boards, only one side wall of about 7m is still uninsulated original brick.
result? Disappointed.
I did not live in the house before the build so can only compare it to the newbuild SAP B flat I moved from. That flat stays warm until January at the very least while I am already using heating in the house in September.
Still feels draughty. All new double glaze sash windows, 2 bedrooms out of 4 fully insulated internally but not warm in September!!
so don't go crazy on the extension, it will make wry little difference due to the rear of the house
I used full fill mineral wool for the cavity as I did not trust the builders to use celotex without gaps.