Hi everyone,
As often the case, this problem has probably been covered in previous threads, but (a) building practice seems to be evolving at a fair pace and (b) individual cases always differ slightly; so I’d like to go ahead and pose the following:
How should we best construct an attached garage and utility room (all single storey, and flat-roofed) to the main body of our two-storey new build masonry house, in a robust manner that minimises thermal bridging and maintains airtightness?
The image attached shows the relevant portion of our plans. The house is to be single skin blockwork, externally insulated in EPS (and finished in render). In principle, we could carry the external insulation around the utility room and WC, but enveloping the garage seems a retrograde step as it would introduce the garage door into the ‘thermal boundary’. To complicate matters further, almost all of the side wall abuts the fence of the neighbouring house (with their property immediately up against the other side of this fence), making external insulation difficult (impossible?).
My instinct is to place an external door between the main house and the utility room and to treat the entire single storey element as distinct to the rest of the house, thermally speaking (and perhaps aesthetically too). I’m even considering building (and externally insulating) only the two-storey house first (neatly preserving the main dwelling as an independent, thermally efficient, build) and then constructing the single storey element afterwards. I’m thinking that the utility room might then best be insulated internally.
Alternatively, timber frame might provide an answer to the problem of constructing the wall against the fence, since the exterior face could be fully insulated and finished before erecting. If we go down this route then I’d like to hear from those more knowledgeable than I am as to how best to integrate the timber frame with the masonry construction of the main build.
All thoughts/questions welcomed.
Thank you.

Thanks for the quick response Tony - I only recently discovered this forum, and already it seems legendary!

Re your flat roof comment: I think (hope) it's just my poor terminology. We'll be designing in a good fall and plan on using one of the regularly recommended (i.e. expensive) single ply EPDM suppliers.

Re the single form of construction, I think I'll take your advice and drop the crazy timber frame idea. But to be clear on the adjacent property, even if I remove the fence, our neighbour's house is merely inches away on the other side. Hence my suggestion that EWI along that wall would be impossible via the usual method of application. I will try and draw up my (possibly equally crazy) "externally insulate the house, internally insulate the utility, don't insulate the garage" idea and post it for comment.
Thanks again.