One-sided insulation/semi-detached house, approval required

Hello everyone, I have a question for you. I am the owner of a semi-detached house=Doppelhaushälfte. Both semi-detached houses are located on one property, everything is regulated by the declaration of division. Now the neighbor wants to insulate the outside of one side of his semi-detached house. I forbade him from doing this because it would look "terrible" and I fear "problems" for my semi-detached house. Unfortunately, the declaration of division does not say anything about this, i.e. whether he is allowed to do this without my permission! Does anyone perhaps know about this? Regards, Monk

Now the neighbor wants to insulate one side of his semi-detached house from the outside. I forbade him from doing this because it would look "poor" visually and I fear "problems" for my semi-detached house.

What exactly problems do you fear?

Unfortunately, the declaration of division says nothing about this, i.e. whether he may simply do this without my permission! Does anyone perhaps know about this?

Unfortunately, you say nothing about the age of the semi-detached house, nor about why you want to forbid your neighbor this? Kind regards

It just doesn't look very good visually, and after re-examining the TE, it turns out that projects on the property and the house are only feasible if both owners agree. And asking differently: Can there be negative impacts on my half? Year of construction is 2004. Regards Monk