About the garage

A word ... that is not true, because it is a problem.
The simplest is to build it beside, just against the house, but separated from it on the whole joint surface with 10cm of polystyrene, that is to say ensurethe continuity of the insulation without fault. I give opposite a possible realization (it is the one I practice). This gives the possibility to create an entrance airlock.

If we want to incrust the garage to the house, we have to include it in the insulation but with a garage door the most airtight and isotherm possible. This will be the comfort of the handyman. It will be good to also foresee an isotherm door between the garage and the house, possibly to insulate the adjoining wall with the house and the ceiling if there is a room upstaires.

In other cases, you'll have to vet the thermal briges, and there will be some !

The cellar garage would not pose too many problems if there were not the stairs. The garage as to be considered as outside the house and the walls and the ceilings have to be insulated like the outer walls of the house, put a wood stairs and an isotherm door upstairs. If it is in masonry, it should be wrapped in a box and with an isotherm door at the bottom. If the stairs is in a part of the cellar apart from the garage, there is nothing to add, it is enough to insulate the garage well, "internally", that's the last straw, from the rest of the cellar, the outside walls being insulated on about a meter deep.

Note: if you have a burried and close cellar, it is sufficient to insulated the foundations on a meter deep like for a house without cellar. This would make an excellent insulation airlock. A slight ventilation should be planned.