Insulation of a detached semi-detached house - experiences?

Addendum:
In the meantime, I asked the client for the disclosure of a comparative calculation showing how much the difference in insulation performance is when insulating the affected wall with only 6cm instead of 16cm, possibly using a higher-quality insulation material.
The answer was not very detailed:
The building meets the standard of the Energy Saving Ordinance (Energieeinsparverordnung 2016 DIN 4109).
According to the building description, 6cm WLG 035 is planned for the wall in question.
Contrary to the Energy Saving Ordinance (60mm insulation board WLG 035), a 60mm insulation board WLG 022 can be installed.
This would mean that an insulation value of about 60% can be achieved.
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Can you make sense of the insulation material designations? Are the 60% a realistic figure? I interpret an insulation performance of only 60% to mean that I will definitely incur significant additional costs.

How would you proceed further?
Thank you all

Can you make sense of the designations of the insulation materials? Are the 60% a realistic figure?

Purely mathematically: 6 cm insulation with WLG 022 achieves 59.66% of the insulating effect of a 16 cm insulation with WLG 035 [(6cm : 022 W/mK) : (16cm : 035 W/mK)]

The questions are rather, what does this mean firstly for the overall energy demand of the house? Probably not much. And secondly: Could this cause moisture problems on the respective interior walls because they are then colder? I don’t know. It also depends on the material of the masonry.

Thank you. So at least the 60% is correct. The affected wall, like all other exterior walls, is made of calcium silicate brick.

But I assume that the owner does not want to sell.

Me too. The owner will hardly make his building plot unbuildable by selling 1m, 3m, or 3.5m.

I think I have the best chance to negotiate a right of first refusal with the owner. There is even the option of a real right of first refusal, where the priority to buy is registered in the land register.

Why should the neighbor agree to that unless he gets paid handsomely? And if he wants to sell to a good acquaintance anyway, simply a particularly high selling price will be agreed upon, which you will politely refuse. Also, it does not solve the problem of the insulation.

If I now insulate with 16cm, I exceed the limit, which means a building encumbrance that I would have to dismantle at my own expense if the neighbor sells after all and I have no right of first refusal.

You have a building permit for a boundary wall. Once everything is finished as approved, you can insulate the remaining 10cm without a permit according to §65 (2) 2. Building Code NRW. The use of the 2m² from the neighbor’s property is contractually agreed with the neighbor, including an obligation to dismantle in case of neighboring construction. Then you do not need a building encumbrance or an entry in the land register. And the costs for the dismantling are certainly lower than a subdivision or a right of first refusal.