2.25m below street level - Cost estimation

I would clarify with the building authority at what level they consider the "ground surface edge" in terms of reference measurements. Sometimes, textual stipulations in the development plans also specify where and how something should be filled or leveled.

So, the idea of "bringing my property to street level with fill" doesn't have to be "correct." It's better to ask one time too many.

We often have this issue here: building a garage on the boundary, it may be up to 3 m high, but from where is it measured? The more differently the neighboring properties are elevated, the more heated the discussion becomes. So coordinating with the neighbors isn’t a bad idea.

Just for orientation: Under our house, which measures 15x15.5m, we had to remove one meter and then add up to 2 meters in total. Additionally, there is the 100 sqm driveway and the excavation went into the garden. We ended up at 25k.

Are you the only ones sitting in the hollow with the neighbors around you being higher up? With such a steep level difference, I would almost build a basement. That was recommended to us as well, but we absolutely did not want a basement.
The area under the house and the driveway is filled with gravel. If permitted, RCL gravel (requires approval). The garden should be filled with fill soil and on top a nice thick layer of topsoil.

In the development plan, in the paragraph regarding the eaves and ridge height, it is clearly regulated that the top edge of the street surface is to be taken as the reference measure. There is nothing about a terrain top edge in the development plan.