Question about estimating the renovation costs of an old building

I would get someone who looks at this holistically. Especially because two of your measures might be of an energetic nature (roof and new windows). That should fit together. Especially with the current building fabric. Otherwise, you'll have more trouble than joy from the whole thing.

I hope you're not referring to the myth that better windows automatically lead to mold... The cause of mold is the underside of the dew point on the wall. This can be countered with proper heating and ventilation. Forced ventilation due to leaky windows is eliminated. But as I said, the KfW and also the BAfA support various things including burglary protection. There are also grants, for example for energy consultants. I would urgently recommend getting information here!

I hope you are not referring to fairy tales

My remark refers to the fact that the different measures should not simply be commissioned individually, but should at least conceptually fit together.

That is only one aspect of a possibly increased risk of mold due to energy-related measures.

Thank you very much for the hint, I hope that with new triple-glazed windows and a newly insulated roof we will manage. I definitely do not want to insulate the 60cm slate exterior walls unless I am forced to do so by the energy regulation. I believe this would be the case if more than 10% of the exterior plaster or masonry needs to be repaired/worked on. But that is not the case for us, the house was repainted less than 10 years ago.

Regarding the windows:

There are a total of 11 pieces with roller shutters (manual, will remain), some of which have special sizes (4 pieces approximately 117cm x 170cm). Luckily, we still have the invoice for the current windows, which were installed in the 70s, with the work done and all measurements. The same company is supposed to do the installation as well. We roughly estimate about 800 to 1000€ per window, so 11,000€ in total. On the ground floor (6 pieces), security windows are to be installed.

We still have to figure out whether wood, aluminum, or ultimately plastic for cost reasons. We'll see.

I'm trying to reach the electrician today so that he can give me a rough estimate for the electrical renovation; he has been working on it for two weeks now...

I am looking here in the forum for experience reports where people mention renovation costs. They never match 100%, of course, but there are always suggestions to be found.