Extension building issue

[Hothammock]

We are trying to come to a resolution with our architect and builders about constructing our extension.
The new extension adds onto a part of our house which is single block construction and has no insulation. We are upgrading this to add insulation.
The architect spec was for external insulation on single block construction. However in the real world this presents issues as the builder can't access all the walls to apply external insulation (existing structures) and there would also be a step out for the part of the house with external insulation added and the old part of the house without it.

The builder has suggested a solution of switching throughout the building work to internal insulation on the solid block walls.
The rooms are large so we have the internal space for this.
The architect is not happy and says it is a retrofit approach and not ideal and we should insist on external.
I am irritated by the whole thing and don't really want to be involved in this kind of detail as that is what I am paying these blokes for!
Help me this is keeping me awake.

If you can’t get the external insulation on certain walls , I’d go with the internal option....... simple.

Well...the architect is right in so much that's the best way to do it, but if you can't physically access to do the work, they are being ridiculous. I would arrange a site meeting with them together as I cannot imagining a qualified architect specifying what they have when it is j possible to construct

The architect is a twat. Your builder is offering a compromise because he cannot access the external walls.

You as the customer need to out your foot down and instruct both what you want doing.

There is no best way, both are effective. Internal method is cheaper but if the builder cannot access the walls then the decision is made for you.
If the architect has his knickers in a twist it's tough luck, he needs to work the problem not sulk. Tell him to earn his money and make a new proposal or suck it up and accept the internal method