Garage conversion for extra room

[BlueLoo]

Thank you that seems reasonable.

I think the width is 2.35m atm so 2.2 is after conversion and there is only one external wall (apart from the garage door into a wall and window.

If its a modern build, you should still have the same wall construction as the rest of the house. Tap the external walls, or check behind the plug socket, you may find its insulated plasterboard, thus no further insulation is required.
No attic will make it much warmer (lucky you!)

In fact, do it yourself. Its easy if you take your time.