Garden wall 105m² up to 2.5m height as enclosure - which system?

More than 1.8 meters fence height is not allowed without a building permit.

That is not true.
It depends on what is stated in the respective state building code.

In Lower Saxony and Bremen it is 180 cm.

Should the wall be built instead of a slope? Then pressure is exerted on the ground on one side? Or how is that supposed to be understood... maybe I just don’t understand it... At least one would have to consider a frost-proof foundation in that case.

But the state building code — that is basically always valid and should/must be complied with. Therefore, one should not just let people talk here and reject good advice, but also listen and think.

Mobile greetings from on the go

It is completely clear that the state building code applies. It is equally clear that the project, if necessary, will also go through the official approval process. However, the building authority will hardly refuse permission if the neighbor, for example, assumes a construction easement, if required.

I am not concerned here with going over any approval regulations.

What interests me is how I can best and most cost-effectively build a wall that holds back the embankment. The property slopes about 3.5-4.0m diagonally. Almost half of the slope has been compensated by "building into the hill." The other half will be compensated by an embankment.

However, making this embankment with a slope is out of the question due to the high space requirement. Therefore, a wall is needed that I can backfill.

What interests me is how I can best and most cost-effectively build a wall that holds back the embankment.

With L-shaped stones.

We have a similar problem. However, only up to 1.20m high. over 42m long

I have tried all kinds of things. From L-shaped stones to industrial masonry techniques.
The following methods were the most cost-effective:
1. Build the formwork yourself and pour: You build formwork over 2 to 3 meters yourself and pour in the mixed concrete including reinforcement. You can pour the foundation at the same time.
Advantage: it is the cheapest and you can work in stages
Disadvantage: It is a tedious job to constantly adjust the formwork and you have to like the look.

2. Dig a concrete foundation in the sheds (plus reinforcement upwards) and then lay formwork blocks on top, which you then fill with concrete - of course mixed by yourself.
Advantage: You can work in stages and the result actually looks good
Disadvantage: The quantities of formwork blocks sit around for quite a while and those things are not exactly light.