Frost skirt under existing or planned terrain?

A frost-free foundation, which must be approximately 0.80 to 1.00m deep - must it be 0.80 to 1.00m below the original or below the planned ground surface?

Reference height is the planned top edge of the plot. The frost-free foundation depth is defined by the soil survey with the classification of the subsoil into F1 (not frost-sensitive), F2 (low to medium frost-sensitive), and F3 (frost-sensitive / very frost-sensitive). It varies regionally; in Germany, it is at least 80cm.

Before I start a new thread about the frost skirt, I’m going to ask for advice here directly.
Assuming a foundation cushion of 0.7m below ground level is sufficient (soil replacement) and the building is done with a frost skirt and without a basement. According to the soil report, the frost skirt should be 1m deep from the finished ground level. However, the underside of the slab equals the finished ground level.

1. If I execute the slab (20cm) finished top = finished ground level, is a frost skirt of 80cm sufficient? Overall, you then reach 1m including the slab.

2. Does the excavation contractor do the excavation for the frost skirt along with the excavation of the foundation cushion? Or do they only excavate a flat area of 0.7m x width x length and compact the gravel? And later, during the construction of the frost skirt, does the slab contractor excavate down to 1.1m? I find option 1 more complicated and have never consciously noticed it. The background is that, according to the construction specifications, the soil in the frost skirt area should be hand-excavatable for the slab contractor. But that does not make sense to me if they would have to excavate an additional 0.4m of soil.

Hi, 1. Yes, that is correct. Of course, other problems can then arise if you "sink" the base slab into the ground (water). 2. Yes, the normal procedure is that the earthworks contractor creates the area and the frost skirts are only excavated by the next trade. If 00/32 or 00/45 gravel is installed, a mini excavator is of course necessary for this.

Thank you very much. Water is actually not an issue. After 1.70m there is gneiss and no groundwater up to that point. The house stands on a slight slope (0.3m over 10m).

Groundwater is rarely the problem. Surface/slope water, especially after heavy rainfall events. If it flows towards your house, then there is no longer a 20 cm threshold but it flows into the living room. Then the question is how your floor slab is sealed. If water can run against it or accumulate, sealing according to W 2.1-E (pressing water) or W1.2 E (drainage) is necessary.

Of course, all of this is feasible. The easiest way is of course to excavate the frost skirts to a meter.

Alternatively, you can also create a gravel cushion of 1.0 m in F1 quality (frost protection) and omit the frost skirts. You have to calculate the costs for that. (30 cm more excavation and filling versus frost skirts and additional effort)