Cost estimate for renovating a 1970s semi-detached house according to KfW or BAFA

I formulated "insulation top floor ceiling" poorly or unfavorably.

But what did you mean? – the ceiling above the upper floor is not sufficient, even if you were willing not to include chamber/storage, because they are accessed via a stairwell instead of just a hatch. So the attic including the slope must be within the thermal envelope; only the cavity under the ridge can be left out, in that sense this strip is your "top floor ceiling."

or maybe roof insulation after all?

Beware of semi-detached house! – if the neighbor does not do it the same way, it will be tricky.

Maybe a lot of electrical lines can run through the floor within the scope of the new floor construction.

I see an underfloor heating rather routed by milling (?), the consequences of a new floor construction on heights (parapets, lintels, stairs) are often overlooked.

We would rather not move in there during construction with 3 children (14y, 4y, and 1y). We accept the double burden for that time.

That continuing construction during already started occupancy is possible is demonstrated to you here by with a somewhat older house:

No. You don’t see that. It’s a prejudice. Feel free to come by and/or I’ll take you along to construction sites of our clients.

Thank you for the offer, but I think we are geographically a few kilometers apart. Professionally, I have nothing to do with construction, but as a private person, I build disproportionately often and with disproportionately much own work. How is it done in detail then? Are the chase holes filled up and then the whole wall plastered and afterwards wallpapered? Or really just the chase holes filled up and then trying to match the fine plaster to the existing surface? Because I imagine the latter to be quite tricky, especially as own work. I once did that for an acquaintance who had bricked up a door, and I then tried to plaster it shut and match it to the existing surface. Well, it was so-so... But it was clear that a wardrobe or kitchen would be placed in front, so the result didn’t have to be perfect.

How is that done in detail then? Slots closed up and then the entire wall filled and afterwards wallpapered?

DIY: Rotband or similar (depending on the wall and plaster) to roughly close the slots. Then fine filling (and sanding). Whether the entire wall has to be filled depends on how the wall looks after removing wallpaper. For me personally, only partial filling was necessary. Afterwards actually even without wallpaper, just a fine finishing plaster (but not one from the hardware store). Painters then fill the entire surface so that a 110% even appearance is achieved and the existing plaster is also "trimmed to new."

But what did you mean? - the ceiling above the upper floor is not sufficient, even if you were willing not to include the chamber/storage, because they are accessed via a stairwell instead of just a hatch. The attic including the slope must therefore be included in the thermal envelope; only the cavity below the ridge beam can be omitted, in that sense this strip is your "topmost floor ceiling".

So I mean it is not enough to insulate the ceiling under the loft. The roof structure is intact. I think it has to be done this way if I understood the expert correctly and the 40x40 hatch can then be closed.

I think it should be done like this,

You don't need to fill the cavity. Whether intermediate joist insulation or (loosely laid) raised joist insulation is used can probably be chosen according to preference.

DIY: Rotband or similar (depending on the wall and plaster), to roughly close the slots. Then fine filling (and sanding). Whether the whole wall needs to be filled depends on how the wall looks after removing the wallpaper. For me personally, only partial filling was necessary. Afterwards, even without wallpaper, just a fine final plaster (but none from the hardware store). Painters then fill the entire surface to ensure a 110% uniform appearance and the existing plaster is also "trimmed as new."

Well, that's how I imagined it or have done it myself. I think you have to estimate what makes more sense depending on how it looks after the new installation.