ᐅ House with Garage Built in a Depression, Elevation Determination

Good evening,
we are planning to build a prefab house on a slab foundation with an attached garage in Hesse. We have not yet had a meeting with the architect, but we would like to gather some information in advance regarding the height determination.
The plot is located in a sloped hollow but is almost level itself. The average elevation of the plot is approximately 138.95 m (456 feet). The top edge of the slope has a height of 139.53 m (458 feet). The sewer connection will be between two manhole covers at heights of 139.52 m (458 feet) and 139.86 m (459 feet).
Next to the house, a garage with direct access to the house is planned. This will be built on the side property boundary using strip footings.
Our current assumption is that both house and garage will be founded at the natural ground surface level. Since the slab foundation and the house floor will be elevated, and we plan to fill the area around the house accordingly, the entrance from the street will be approximately level with the house entrance. The driveway to the garage will have a slight slope.
Questions:
1. Is there any reason not to proceed like this, or are there better alternatives?
2. As far as we know, foundations have to be placed on undisturbed natural soil. Would it be possible to place the strip footings for the garage in the fill soil, ensuring they reach down to the natural soil, in order to raise the garage?
3. We assume that due to the elevation conditions, we will need backwater protection. Are we correct in this assumption?
4. Should the slab foundation be made of waterproof concrete (WU concrete) if we are backfilling around it and it lies below street level, or is standard concrete sufficient here?

Thank you very much for your assessment.

Is there a geotechnical report for the building site? An important factor for determining the height would be having knowledge about the soil and water conditions on, in, and around the property. Although no basement is planned, the foundation of both structures (house and garage) must be adapted to the ground conditions.

It is somewhat difficult to accurately imagine the actual conditions and provide correct answers based on them.

The question of using a waterproof concrete slab depends largely on factors such as the design water level.

All building openings below the backflow level must be secured. For example, in our case, the geotechnical report specified exactly where this backflow level is located.

Is your property situated in a depression on all sides, or can water drain away somewhere during heavy rain? How large is the area from which water flows onto your property during a storm? These are the most important questions to start with.
The garage foundation can easily be raised by a few centimeters by adding and compacting gravel accordingly. I would definitely install a backwater valve, ideally inside a small inspection chamber (for example, a large PVC drain pipe) located outside the building. This way, it can be accessed if needed.

A plan showing the heights and slope directions could be helpful.