Thanks a mill
@Micks'r for such a brilliant/detailed answer.
This is all really valuable when we sit down with architect to ask him (slightly) informed questions to make sure all things airtightness, ventillation, insulation, heating are considered and thought through. Current planned architect is very bright, not certain how good he is on specifications for insulation, ventilation & airtightness - we just haven't got into the detail on this, but I have seen his drawing packs for builders and they are very detailed. If he doesn't want to get into the detail on this I can always get additional professional(s) to advise.
Re airtightness - I understand the importance....though until now I had kind of been ignoring a scientific approach as am conscious that chimneys & front door with single pane stained glass side panels and window does not play well with low air tightness scores. Herself is not keen on a porch to the front door so this is a challenge re airtightness.
I figured doing the floors, good windows, attic etc makes it much better, and was going to leave it there. But I take your point that being able to answer questions on heating is dependent on the approach to airtightness, amongst other things.
Re chimneys....we'll probably end up with only one "in use" fireplace. The other chimney downstairs will become a stove, to be confirmed. Other chimneys are in 3 bedrooms, currently covered over (metal plate in fireplace opening to chimney).
I'm also quite concerned about thermal bridging and overall decisions on insulation in relation to this, as you have alluded to. We have solid concrete soffits/fascias, so that adds complexity I believe. We dont have any insulation on walls, internal or external. We also have "hip roof" I think it is called, another complexity. I figure by converting the attic, as well as gaining valuable space, we also gain by dealing with any airtightness/insulation that needs to be addressed.
I'm not 100% sure what you mean re build passive for the extension - the extension is largely half of an open plan kitchen/utility area, the other half being one of the current reception rooms.