Who has to support the property?

I only know it as the one with the higher property pays, but often an agreement is reached to share the costs.

And if someone lowers their property afterwards, does the higher neighboring property owner still have to pay? Hardly. In my experience, the person who initiates the change is the one who pays - including the responsibility for the costs resulting from it.

Right, that makes sense. So whoever changes the level in one direction is also responsible for the costs from the start. However, in practice, people will usually come to an agreement, for the sake of good neighborliness.

Lord, that is still coming our way.

Although I think the neighbors—in our case—must take responsibility for that.

In the development plan applicable to us ([Änderung erfolgte erst im Frühjahr 2014]) it stated regarding height determination:
"Fillings and excavations contrary to the natural terrain are not permitted."

We are located on a slight slope. Our finished floor level is 1.50 m above street level. The ground in the garden rises further.

However, our neighbors to the south have excavated down to street level; in a phone call we had at the very beginning of the planning phase, the neighbor additionally mentioned that they would go 80 cm lower so that two full floors would fit.

So now they are looking at a 2-meter earth wall behind the house at the terrace. We have adhered to the natural terrain and I do not see why we should bear any costs caused by the excavations.

In the meantime, the development plan for our building area has probably been changed on this point. However, only in spring 2014, by which time we were already included.

I am curious what else is still coming our way ...

I am also curious to see how things will continue with you.

There is news from my side as well.

Not my neighbor, but the city is opposing the retaining wall. They only allow a retaining wall of about 3m (which the neighbor has already built) - no more is allowed; the neighbor apparently wanted to build a longer wall on his own in order to have parking spaces for his cars directly on the street - now he can only park his cars parallel to the street - quite annoying!

Therefore, I redesigned my terrace and my basement so that I adapt to the course of the property and my terrace will then be on ground floor level on the north side (and not on basement level as previously planned). It fits better anyway, since later when grilling I won't have to walk through half the house from the kitchen to get to the terrace.
A privacy screen will then "only" have to be installed towards the neighbor, but we'll sort that out later.

: I'm keeping my fingers crossed that things will work out positively for you. Looking forward to seeing how things develop for you.