Maybe I'm naive, but of the endless choices I need to make in this project, I hadn't given the specific type of insulation a great deal of thought. I currently live in an old cottage so if I'm sat in 15 degrees now, I'll be lucky (storage heaters and log burner). Unsure what you mean by permanency? I didn't expect you'd see any difference once the building is finished, whether blocks, timber frame etc. Clad on outside and boarded and plastered inside.
I have a friend who is a well respected local builder - they chose a SIP construction for his son's house. No complaints with the method, construction etc. They will be a reasonably affluent family and, as builders, could have built it in any way they chose - subject to meeting the look required by the planners in the parks.
I hadn't considered the concrete filled blocks, no. The extension stands as a separate building, joined by a short glazed corridor. The planners wanted clear separation of the new and old. It will be a shared airspace but two distinct spaces.
The long game is to build a house to live in, it's at my place of work, has a huge workshop behind, 1 hectare site. Be happy living here 'forever' or until too old and knackered to look after the place. I have two young boys and with housing costs, I want them to have lots of space - it's madness building a 3 year old (5 if build competed on target) a room with an en-suit - but if housing prices continue as they are, they might be at home for a while. The boys rooms, in the extension, mean they can have some freedom/privacy as they get older, and blast some music without it affecting me so much.
The stone building is well-built but will be completely insulated, and hopefully airtight to modern standards. Some large areas of glazing will increase heat losses, but they are a lot of the character of the building (two large arched doors where trains drove through the shed).