Renovating an old building, which insulation measures are mandatory?

We have 60cm thick slate exterior walls,

Please post some pictures of that. How certain is this finding? - Slate as an actual load-bearing building material for monolithic walls is known to me, but even considering regional differences, it is rather not typical contemporary for the construction year 1930. The Pellenz or Vulkaneifel are full of houses that have a thick facing shell made of slate (or here more frequently: basalt), but behind that they are built of pumice stone. Along the Mosel, it should behave similarly – with the variation of a different stone material. In any case, for the construction year 1930.

Thanks again!

The designation "slate wall" comes from the current owner, who has lived in this house all her life; her father had the house built at the time. Original construction plans from the building period as well as all remodeling/extension plans and (repair/renewal) invoices from the following years are fortunately still all available. A thick stack of paper, but very useful to be able to understand a lot. Repairs were always carried out immediately.

Fortunately, we already know the house extensively, as mentioned, it has been in the family. We can enter anytime to inspect or take measurements, even if craftsmen have to come in to take measurements for cost calculations.

This has a huge advantage compared to a house that you usually buy from an outside party. There you have to have a rough idea of the renovation costs after one or two viewings, and the follow-up costs due to "surprises" should not be underestimated either. With luck, you have an expert or architect with you, ideally even an appraiser.

Finally, the owner's statement about the exterior masonry: ground floor and 1st upper floor slate masonry (according to the 1930 building plan 60cm thick), the floor above 2nd upper floor pumice stone masonry, 50cm.

As for the roof:

It currently still has 2 dormers, which are to be removed later (in consultation with the building authority) and replaced by Velux windows. Dormers are not cheap either; for our purpose, we don't need them anyway (open roof truss).

Otherwise, two very ordinary roof shapes without much frills. What I already know so far: the price difference between internal insulation and external rafter insulation is enormous, external insulation costs almost twice as much. We still have to consider this: open roof truss = external insulation and "finer" woods inside (since visible), or insulation inside and cladding the beams with drywall.

Fortunately, we still have at least half a year for idea gathering and awarding contracts. We also calculate a good 4 months for the renovation, even if we do as much as possible ourselves.

We will leave the electrical work and the roof entirely to the companies.

In conclusion, the owner’s statement about the exterior masonry: ground floor and first floor slate masonry (according to the 1930 building plan 60cm thick), the floor above that second floor pumice stone masonry, 50cm.

That cannot be ruled out, but for this year of construction it is rather unusual. But you don’t have to believe anything, you can know a lot: you can evaluate such assumptions. With every little hole you drill into the wall. Window reveals tell all kinds of things, especially what was seen there during the last window replacement. Thermographic images are also good clarifiers.

It currently still has 2 dormers, which are supposed to disappear later (after agreement with the building authority) and be replaced by Velux windows. Dormers are not cheap either; we don’t need them for our purpose anyway

You only stand on the "parapet" of a roof window when it is open; a vertical window gives a completely different view. That should not be underestimated. The dormer itself breaks the wind a bit, which is different with roof windows. When insulating, of course every roof structure counts as a "disturber" in the "overlay."