ᐅ Frost protection barrier below existing or planned ground level?

This is a slab-on-ground made of waterproof concrete. According to the plan, the top edge of the slab will be 7cm (3 inches) above the planned terrain level (which means the frost protection apron is only 93cm (37 inches) deep). On top of that, there will be an additional 18cm (7 inches) up to the finished floor level. Additionally, we are located almost at the highest point in the area; the nearest higher point is only 1 to 2 meters (3 to 6 feet) away within a 500m (550 yards) radius. I believe/hope that the 7cm (3 inches) height of the slab above ground is sufficient. If water were to enter the house, the entire village or even the whole district would be flooded 🙂

The side that could be affected by hillside water runoff also has a terrace, where additional frost protection is installed across the full width (10 x 3m (33 x 10 feet)). This should provide extra buffering.

The frost protection apron of 80cm (31 inches) is mandatory by law and is always included in Heinz von Heiden constructions. We are also fine with this because it means there is only one step into the house.

The small-scale terrain around the house is crucial first of all. If rainwater is properly drained there, 7cm (3 inches) should be sufficient.

Attached is the side view again. I have three questions:

1. The civil engineer is responsible for the foundation up to the underside of the slab-on-grade, correct?
1.1. Am I then responsible at the end of the construction process for creating or commissioning the blue area (13cm (5 inches) fill up to the planned site level)?
2. The green area is excavated by the frost protection footing contractor, right?
4. Is there a blinding layer beneath the slab-on-grade? We plan to use certified recycled concrete aggregate (5-45) or fine stone sand (0-32).

Feel free to help me with calculating the excavation volume and gravel quantity. The working space is 2m (6.6 feet) with a 65-degree slope. Length 11.33m (37.2 feet) x Width 9.08m (29.8 feet). According to the soil report, the humus layer needs to be excavated down to 0.5 to 0.7m (1.6 to 2.3 feet).

- Current ground elevations at house corners are: Corner 1: 382.0m (1253.3 feet), Corner 2: 382.0m (1253.3 feet), Corner 3: 382.1m (1253.6 feet), and Corner 4: 382.6m (1255.3 feet)
-> I calculate an average excavation depth of 0.675m (0.5 + 0.5 + 0.6 + 1.1 divided by 4), assuming no stepped excavation.
- 15.33m (50.3 feet) x 13.08m (42.9 feet) x 0.675m (2.2 feet) = approx. 135m³ (4766 cubic feet)

Gravel fill. I calculate an average fill height of 0.57m (1.9 feet), rounded up to 0.6m (2 feet).
- 15.33m (50.3 feet) x 13.08m (42.9 feet) x 0.6m (2 feet) = 120m³ (4238 cubic feet) plus 10% compaction = 132m³ (4662 cubic feet)

Is this roughly correct, or am I making a mistake somewhere?