Rough estimate of renovation effort / costs

I had read, however, that insulation in the 70s was not yet optimal and that building materials were also more toxic (asbestos, etc.).

Of course, the insulation was not yet optimal. From today’s perspective, it wasn’t in the 2000s either. And certainly not in the 1920s. That’s why these properties are also cheaper than new builds. And toxic building materials is a completely exaggerated description. Asbestos, lead pipes, etc. are actually not such big issues (asbestos was used from the 50s until the 80s, but the peak period was indeed from 60-80).

Naturally, the thermal insulation was not yet optimal. It wasn’t in the 2000s from today’s perspective either. And especially not in the 1920s. That’s why these properties are cheaper than new buildings. And toxic building materials is a completely exaggerated description. Asbestos, lead pipes etc. are actually not such big issues (asbestos was used from the 1950s into the 80s, but the main phase was actually from 60-80.

How do you find out if there is asbestos in there? Building specification? I’m just asking where it would be located in that case? Roof/walls?

What was generally bad in the 70s?

In the 70s there was a construction boom similar to today, also with comparable rates of poor workmanship. But you can look at that calmly: fifty years should be enough for construction defects to become visible.

How can you find out if there is asbestos inside? Construction description? I'm just asking where it would be located in case? Roof/walls?

By carefully taking the house completely apart. Asbestos was used in about 3,000 products. In house construction often in panels – roofs, facades, shafts. It can simply stay where it is, as asbestos in bound form does no harm. It was also often used in adhesives, coatings, or similar. But you can't recognize those yourself. Or it was processed in particularly contaminated plasters – e.g. base plaster. Honestly, I would never think seriously about it, since asbestos in the house is mostly harmless. Why? It is mostly firmly bound in the material and does not release any fibers. Because only the fibers are the problem. Therefore, there is also no obligation, for example, to dispose of asbestos roofs or facades.

That's how it looks, asbestos in a bound form is virtually harmless. Therefore, it is also worth considering whether to leave it where it possibly is. However, as far as I know, building over it is not allowed. So, if the facade contained asbestos, you would not be allowed to "hide" it behind another one.

Because if asbestos is released, it can be very dangerous. When the fibers are inhaled, they are alveolar-penetrating and thus enter deeper into the lungs and can in the worst case lead to lung cancer. There are also other lung diseases like COPD, etc., which asbestos can cause even at lower exposures. As with all hazardous substances, there are also people who were massively exposed to the hazardous substance partly unprotected and have virtually no impairments, but one cannot/should not/must not rely on that. One should proceed with the necessary caution when dealing with asbestos, but as long as it is bound in a material, asbestos itself is "harmless."

Got another question. I asked about insulation. The semi-detached house has an energy efficiency value of 115. But that apparently includes everything. So also photovoltaics and solar water heating. Is that acceptable? The other house (terraced middle house) that I have provisionally reserved has a value of 45. Maybe you can still improve something if you replace the heating at some point. But I think the facade is not insulated.