No, I do not know of a plaster which comes anywhere near as effective as conventional insulations

It could be that he plans to use a very thick insulation laminated plasterboard or suchlike. Even so, there are structural implications when only having a single skin masonry wall. I expect the Building Inspector will have something to say

You can do a weighted average U-value calculation for the whole house plus an extension built according to the U-values in Mike's post and compare it to the extension insulated how you want to do it plus the house as it will be after the job is complete (e.g. you could up rate the loft insulation in the whole house as a trade off for higher levels of glazing in the extension). I've just done this where I had an uninsulated conservatory slab that I wanted to use for an extension.

The extension I'm building has single skin block walls with external insulation. There are some guidelines in part A of the building regs that set out what thickness of wall is required in certain circumstances e.g. the side walls in my extension are 100mm block but the front wall has to be 215mm because it contains large openings. It is a bit complicated...

No Mike, although there was mention of this at one point. The building inspector just accepted a summary spreadsheet showing a weighted average U-value calculation (along with a copy of my workings). Basically, by increasing the depth of the insulation in the loft in the existing house, I compensated for the lack of insulation in the existing concrete slab. Quite a cost saving so was worth the effort in this case.