"tonyjeffs" wrote in message
...

Is there a thin walling material, maybe 3" thick, that has high
insulation properties, and that might satisfy planning, that I could
use instead?

Assuming you are allowed(1) to use the party wall as part of the structure
then foam backed plaster board should do the trick.

1: You would have to attach the roof and put flashing along it to make the
joint weather proof.