Building a residential/hobby basement underground - What to consider?

Associations in the direction of Kampusch or Fritzl in the local rumor mill included.

I consider that nonsense, no one would build that with approval, would they? I would create a sketch (top view), based on the site plan of the property, scale 1:100. Then one could estimate the space requirement and illustrate the routing of pipes, etc. According to the questioner, a supply line for the heating is already prepared (IIRC); it would then have to already leave the house. Such details ...

This is Germany ... immediately thinking something bad when you do something that deviates from the "norm"

I will create and upload a corresponding sketch showing how I imagine the whole thing.
Until then, thank you very much in advance and I am happy to receive further information

I filled between the wall and the curbstones with filling gravel

I don't understand it here anymore. Do you have photos of your project?

Crazy idea is definitely the keyword here. Because your wife wants to put her car in your current fitness room (garage), you want to build a bunker that will probably exceed the value of the car by several times? Some people just think differently. The height of emotions for me would be a carport for the holy metal, the pragmatic solution a parking space on the obviously sufficiently large property.

The property is sufficiently large. Personally, I would also prefer a glass box in the garden: it must be wonderful to do your fitness in a conservatory or greenhouse. You are not necessarily seen there if it is well positioned and possibly a few hedges or bushes are planted. Opposite then the sauna with outdoor shower and the wellness area is complete. However, there is the risk that the woman will also claim this building for herself. Wastewater is probably not a problem with the underground dungeon, rather with the one I just mentioned.

Here I don’t understand it anymore.

Hello Bieber0815

A brief explanation. (I can’t upload photos at the office)
The excavation pit is larger than the eventual building. Now I have calculated the slab slightly larger than the building to be built on it. Let’s say the building will be 4 x 4 meters. I calculated the slab to be 4.4 x 4.4 meters. And the excavation pit is 5 x 5 meters. Now I limited the 4.4 x 4.4 meters with curbstones, which I set upright (so the 100 cm vertical). I poured concrete into this area. Then the building was built up with masonry. Now I have two cavities. One from the building wall to the curbstones. This space I filled with filler gravel. Then from the curbstones to the natural ground. This was filled with RCL. At the upper edge of the curbstones, after everything was backfilled, I set more curbstones. Filled again. And this three times in total.
Is it roughly understandable like this?

Steven