House with garage in a depression, height determination

Good evening,
we are planning to build a prefabricated house on a slab foundation with an attached garage in Hessen. The conversation with the architect has not yet taken place, but we would like to gather information about the topic of height determination in advance.
The property is located in a terraced depression but itself has virtually no slope. The average height of the property is about 138.95 m. The upper edge of the slope has a height of 139.53 m. The sewer connection will be made between two manhole covers with heights of 139.52 and 139.86.
Next to the house, a garage with direct access to the house is planned. This is to be built on the side property boundary on strip foundations.
Our current assumption would be, at first, that the house and the garage are founded at the height of the natural ground surface. Since the slab foundation and the floor of the house will be raised and we want to fill up the area around the house accordingly, one would come relatively level from the street to the house entrance. The driveway to the garage would have a slight slope.
Questions:
1. Is there anything speaking against doing it this way or are there sensible alternatives?
2. As far as we know, a foundation must take place on naturally grown soil. Could the strip foundations for the garage also be placed in filled soil, so that they reach the naturally grown soil in order to raise the garage?
3. We assume that due to the height conditions a backflow prevention device is needed. Are we correct?
4. Should the slab foundation be made of waterproof concrete if we backfill around it and it lies below street level, or is normal concrete sufficient here?
Thank you very much for your assessment

Is there a soil report for the building ground? An important point for determining the height would be for me the knowledge about the soil and water conditions on, in, and around the property. Although no basement is planned, the foundation of the two structures (house and garage) must also be adapted to the subsoil. It is a bit difficult to correctly visualize the actual conditions and to provide accurate answers based on them. The question of a WU floor slab largely depends on factors such as the design water level. All building openings below the backwater level must be secured. For example, in our case, the soil report made a statement about where exactly this backwater level is located.

Are you in a depression in all directions or can water also drain away somewhere during heavy rain? How large is the area from which water flows onto your property during a thunderstorm? These are the most important questions to start with. The garage can easily be built a few centimeters higher by piling up and compacting gravel accordingly. I would definitely install a backwater valve, ideally in a small shaft (e.g. large KG pipe) outside the building. That way you can access it if necessary.

A plan indicating the heights and slope directions could help.