About the peak

It is like for the garage, there is a rigorous solution, then it is handiwork for each case.

If you have a four-slopes roof, the good solution is to build a caisson in the central part, as shown on the sketch opposite, with velux obviously. Do not forget that in same, it is better that air can flow between the caisson and the roof.

If you have gable walls, it is less neat. You have to insulate the gable walls on both side, for the inside part only on the surfaces which are not included in the converted permises. It would be a good thing to also insulate the wall slice. This seems wacko, but thermal bridges are so.