: Your statement is absolutely correct! Since we have filled in, we are obligated to build a retaining wall in this case. (§9 NRG BW)
: In our development plan, only boundaries to "development facilities" are defined. That means, in our case, the Neighbor Law (Baden Württemberg) and the State Building Code BW come into effect regarding the neighboring property.
The topic of the retaining wall is addressed here, and if I understand correctly, I am allowed to build a retaining wall directly on the boundary (since the neighboring property is not used for agriculture), and with my planned height of 80 cm and length of 26 m, I am well below the maximum height of 2.5 m for a retaining wall, or with a wall surface area less than 25 m². (e.g., §10 NRG, §6 State Building Code)
The construction project is also not subject to an approval process if the retaining wall height is less than 2 m (Annex State Building Code, Section 7 c)
Please correct me if I have misunderstood anything here.
Thanks to everyone for the contributions on this topic so far.