Brian Reay wrote on 10/03/2020 :
One of our previous houses had been extended like that. Fortunately, there
was a crawl space, accessible via a hatch in the garage, and I was able to
gain access. I lined it with foil and insulation. It made quite a difference.

If there is a similar space, it may be worth making an access somewhere is
can't be seen.

He says not, but it might be worth his making some access to add
insulation.

It's not an extension, it is the originally built. I'm surprised that
in the 1960's this was an acceptable way to build homes. He says his
was one of the first, later ones of the same design had to have
insulation added.