Rainwater must be obligatorily infiltrated

An alternative would be simply to set up rain barrels, that also catches normal rain without a cistern.

If it can be proven that the capacity is sufficient for the expected rainfall ...?

Just because infiltration is required, a cistern (=retention) is not mandatory. On the contrary, anyone who has a cistern still needs an overflow, either into the sewer or by infiltration again (albeit on a smaller scale).

The cheapest form of infiltration is simply to let the water flow into a depression or a larger ground area. But you need space for that, which hardly anyone has nowadays. The second cheapest option is a soakaway shaft. Ask the authorities whether that can be approved.

The excavation was not insignificant. But fortunately, we were still able to use our soil well on the property.

Yes, the holes are huge. Here is a picture of my hole in comparison to the base slab. The cistern has 8 cbm

just build a soakaway, maybe costs 500€ and done

Soakaway is the right keyword, but 500 EUR is quite far from the current real costs. Depending on the soakaway, you might also only get it delivered in 3 months ;)

The fact that rainwater must infiltrate on your own property and must not be discharged into the sewer system applies in many municipalities.

The architect should actually take this into account and submit a calculation for the appropriate soakaway/infiltration shaft. At least in our case, it had to be inspected and approved for a fee by the "Lower Water Authority". The designed size ultimately depends on the total amount of sealed surfaces (roof, pavement, terrace...) and must be taken into consideration accordingly.

Yes, the holes are huge. Here is a picture of my hole compared to the base plate. Cistern has 8 cbm

I always wonder if it pays off economically. I had a deep well drilled through an acquaintance. 5" pump, 8.5 m3 per hour, it can run all 10 sprinklers at the same time in summer.

I always wonder if it's economically viable. I had a deep well drilled through a friend. 5" pump, 8.5 m3 per hour, it can handle all 10 sprinklers at the same time in summer.

Most likely not profitable. But you can't or shouldn't simply install or use a well everywhere.

Rigole is the right keyword, but 500 EUR is quite far from the current real costs.
Depending on the rigole, you might also only get it delivered in 3 months ;)

I also meant rigole in the form of concrete rings for soakaway shafts.
500€ would be the material cost for DIY, at the beginning he said he wanted to save, hence the suggestion.