The ground under the garage must be stabilized because the neighbor is at a lower level

Good day,
my husband and I are building a detached house with a garage and now have a small problem. Our plot is much higher than that of our neighbor. We would like to level our very uneven terrain to a uniform height. The finished floor, the garage, and the surrounding garden area should all be brought to the same height so that you can step out everywhere without a step. Only in the house entrance area will there be a step. Now the thing is, our neighbor is not backfilling, and we have a slope of about 1 meter towards him in some parts. This is not a problem for us; it was already approved with signatures in the building application, and we will secure our plot towards the neighbor with L-stones. Our problem is the garage. It is located exactly between our house and the neighbor's boundary. So, it will be built right on the boundary. Now the question is, how can the foundation of the garage, which will extend up to the boundary, be secured? Do you extend the L-stones through and pour the foundation onto them? But you would never get exactly the same height there, right? What can be done?

You will probably have to make quite deep strip foundations, I suppose.

How are you building? With an architect or a general contractor? They would be my first points of contact.

With the architect, unfortunately, that is not much help..

Depending on how large the height difference is, a proper foundation must be concreted; if the architect can't manage that together with the structural engineer and the construction contractor, then they are in the wrong movie.

With an architect, but unfortunately he’s not much help..

Then I assume you mean a draftsman of the general contractor. Think carefully again whether the Pyrrhic victory of the most brutal possible embankment really has to be achieved. If the neighbor can have a whole meter lower level, it seems the street/driveway doesn’t really require such an excessive height.
Also compare this case – here you get the other perspective right away:

and we will secure our property towards the neighbor with L-shaped stones

Maybe can contribute with a reference on how it looks when neighbors hold L-stone battles to put an end to it (?)

Our problem is the garage. It is located exactly between our house and the neighbor's boundary. So it is placed on the boundary.

How did you solve the building regulation issue regarding the 3m height restriction for boundary constructions? Since the planner is aware of the height difference, they must already have solutions for all resulting problems.