ᐅ Semi-detached house – Troubles with the neighbor

Hello dear members,

I discovered your lovely forum thanks to Google. Unfortunately, as is often the case, a problem has brought me here.

We are building an extension onto an existing semi-detached house in Baden-Württemberg.
We are building on a slab foundation. The friendly neighbor built his half with a basement.
We live 70 kilometers (about 43 miles) away from the construction site.
Therefore, with both of us working and having two children, we cannot be at the site all the time.
Yesterday morning, the workers began setting the formwork for the slab foundation.
When I arrived at the site yesterday, my friendly neighbor was already there waiting for me.
He yelled at me quite loudly and was extremely angry.
He said that the slab foundation towards his side was insulated only with Styrodur (extruded polystyrene).
One of the workers tried to explain that in addition to the Styrodur, a 4 cm (1.6 inch) soundproofing wall would be installed along the house.
Still, he did not calm down.
He just kept yelling. He said he would take pictures and that he would refuse to pay.

We have no intention of demanding any money from him.
I was really shocked!!!

Actually, he should be worried, since he built 3 cm (1.2 inches) of his house’s total length over our property boundary.

What is the legal situation here?
I told my husband about it.
He wanted to see a lawyer right away.
For me, maintaining peace with the neighbor is more important.

Please excuse my spelling mistakes and my lack of knowledge. I have two children and need to keep them happy at the same time.

Thank you,
Sandra