Facade insulation for a semi-detached house: what/how must be approved by neighbors?

The OP wrote that he is having insulation done according to KFW. That means he is energetically renovating the existing building to KFW 55 standard of a new building. Halved heating costs are therefore quite to be expected, but the forecast must be made by the involved energy consultant.

No, no, no. Not here with apples and oranges and so on!

Yes, if everything is renovated, you might achieve a 50% reduction. But the cost of 21,000 only concerns the facade (and the necessary roof extension). And since the facade accounts for about 30% heat loss, of which you might prevent 70%, you cannot save 50% on heating costs through that. Rather 20%.
Therefore I have to correct my rough estimate from a few posts ago: ROI somewhere around 80 years. Unfortunately, the insulation does not last that long.

But the OP did not ask that at all. So sorry for OT.

Nah, nah, nah. Not here with apples and oranges and stuff!

...
Therefore, I have to revise my rough estimate from a few posts ago: ROI somewhere around 80 years. Unfortunately, the insulation doesn’t last that long....

But the OP didn’t ask about that anyway. So sorry for OT.

If EPS or something similar, the material will outlive you and probably the rest of the house by many, many years (assuming it was professionally installed). Otherwise, our oceans would soon automatically be free of plastic again

If EPS or something similar, the material will survive you and likely the rest of the house for many many years (assuming it was properly installed). Otherwise, our oceans would soon automatically be free of plastic again

Who knows if EPS will be considered the new asbestos in x years and at some point even have to be forcibly removed from the wall.... But that's beside the point, if I now say that we have wood fiber insulation boards..

With such an investment sum, and the problem with neighbors and boundaries, unfortunately, there is never a 100% solution. I would allocate the money differently...
Heating due? Wooden windows, maintenance-intensive? Roof tiles possibly due?
Cold spots...

Before that, I would consider
New heating
Photovoltaics (possibly with the domestic hot water heat pump – which simultaneously dehumidifies the basement)
New windows, new front door,
Insulating the roller shutter box
....

... the material will outlast you and probably the rest of the house by many, many years

That is true, but can I replaster the house after 60 years without the insulation coming off?

I would first consider New heating Photovoltaics (possibly with the domestic hot water heat pump - which also dehumidifies the basement) New windows, new front door, Insulating the roller shutter box ....

In short: Only when EVERYTHING else has been done AND the money costs nothing would I consider facade insulation.

Well, that's not exactly the economical approach if you do the heating first. After all, the insulation drastically changes the demand for heating energy. Ideally, you do it as a package.
I would only do the heating first if you don't plan to insulate.
Even a door costing 3-5K€ doesn't bring nearly as much energy efficiency as facade insulation. This "screw" is one of the biggest factors to turn the energy demand.

But there are professionals for that.

Above all, it is by far not just about the simple calculation "costs of insulation" versus energy savings. With this argument, not a single ventilation system would be sold, let alone a fireplace or anything else...

1) Comfort gain! Living in a well-insulated house is a huge comfort advantage. Even temperature, no heat/cold spots in the house, no cold radiation through the walls... it is much more pleasant to live there.

2) When calculating economically, as has already been mentioned, one must also consider the value of the house. Hardly anyone wants uninsulated shacks anymore... the value of a house increases through good insulation and thus higher comfort and better energy efficiency...