Level change and cost sharing in Hessen

There seems to be some confusion: in some federal states, a general rule applies to Einfriedungen (commonly fences, hedges, walls) that one of the two neighbors (and: which one) is solely responsible on side X or Y. However, this does not say anything about Terrainmodellierungen: for example, everyone must drain on their own property – and water flows downward, not to the left or right. As the higher-level owner, it is therefore your task to secure and drain your soil masses – even if the fence on the L-stones might possibly be your neighbor’s pleasure.

it also indicates that it is only a help how is filling up and so regulated in your area?

There is nothing about it in the development plan and the building authority says I should inform myself about it under neighborhood law. However, I cannot find the decisive point that says something about the cost allocation

Unfortunately, I cannot find anything regarding cost sharing, so here is the question.

The law cannot answer this question either, as the measures of both owners can cause completely different costs. Clearly, for the party digging out, §909 of the Building Code applies:
Digging
A property must not be deepened in such a way that the soil of the neighboring property loses the necessary support, unless sufficient alternative stabilization is ensured.
For the party filling up, there is a similar provision in the Neighbor Rights Act in NRW, while Hesse refrains from such a comparable provision. Perhaps in your area, it is believed that Building Code § 907 Hazardous installations regulates it sufficiently.
Everyone is thus obliged to do something. Regarding the costs, one simply has to agree. Otherwise, the law of the faster applies.

I do not, however, find the decisive point that says something about the cost distribution

The cost burden is distributed according to the obligation. If the law does not specify a clear bearer of the obligation, then it also does not specify one for the effort and its costs.

...for example, everyone has to drain on their own property - and water flows downwards, not to the left or right. As someone living higher up, it is your task here to secure and drain your earth masses - even if the fence on the L-bricks might be your neighbor’s pleasure.

Yes, I will ensure that the water does not flow over the L-bricks onto the neighboring property.

My main concern is the securing. I am going uphill... no problem, I will accordingly stabilize my elevation. But do I really have to go so deep with my L-bricks that I also secure his lowering of the level? That’s exactly why I am looking for an appropriate clause that regulates this.

For me, it makes sense to simply place a larger stone that supports the full 80 cm (his -40 cm and my +40 cm), it is pointless to have two retaining structures side by side and on top of each other here. But I don’t want to pay for the larger stones just because he changed his level so drastically, because 40 cm high curbstones in front of the garage would be sufficient for me, but due to his lowering, 100 cm high L-bricks have to be installed there, which I am supposed to pay for. So not only are the material costs enormously higher, but an excavator is also needed.

That is exactly why I am looking for a corresponding clause that regulates this.

There isn't one. You need to hire a specialized lawyer who knows the relevant rulings or do the research yourself.

It is pointless to place two reinforcements next to and above each other here.

Correct. But if you don’t come to an agreement, that’s what it will come down to.

just because he changed his level so drastically

So he did change the level? And you too? And how exactly does the terrain slope at the border look now?