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

The insulation including roof extension costs 21K. Heating costs are currently 1300 € per year.

Phew, 1300 is certainly something. But even with this extreme heating load, it will pay off eventually in 35-40 years. (Even later if hot water is included). I would think about that carefully and not let an insulation salesperson make it look better than it is. Roof and windows are already new, I assume? If not, then maybe start there first.

Phew, 21k€ is quite a sum...

As already mentioned before, maybe just insulate the roof, check the windows, check thermal bridges - especially with old shutter boxes...

How is the heating done?
I would first install a photovoltaic system on the roof. Depending on self-consumption, return on investment is between 8-10 years.

Without wanting to champion this, the 21T€ is offset by a substantial KFW subsidy. Besides the halved energy costs, one must also consider that this includes increased property value, a newly done facade, etc.

If you want to sell and/or the facade needs to be redone anyway... Possibly sensible, yes. But the energy costs NEVER halve due to facade insulation. I have already calculated with an extremely generous 40%. In the end, it is probably more like 25%.

However, I do not know how high the funding is. What are the numbers like?

Just out of interest:
How old is the house?
1300 heating costs are not much when you look at other numbers from older houses.
Roughly estimated, costs of 800 € per year would be opposed to this.
The basic price doesn't get any lower either.

However: if I had 21000 available, and the house is supposed to bring me joy for another 50 years, I would also do it, the insulation.

I only know that distances to property boundaries smaller than 3 meters may be allowed afterwards if energy-efficient renovation is carried out. In this case, the positive effect is given priority over the carved-in-stone 3 meters.

If you want to sell and/or the facade needs to be redone anyway... Possibly sensible, yes. But the energy costs NEVER halve because of the facade insulation. I have already calculated with an extremely generous 40%. In the end, it is probably rather 25%.

However, I do not know how high the subsidy is. What are the numbers there?

The original poster wrote that he is having insulation installed 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 will have to be made by the involved energy consultant.

The subsidy amounts to 30% of the costs up to a total of €30,000. In addition, possibly further grants for a new heating system, etc.

And you simply have to weigh costs like an already pending roof/facade renovation.
I think an ROI of 15-20 years is definitely achievable and in my opinion that is not a bad return.