Design of Rainwater Infiltration (Swale)

The depression hides by itself with good garden design. 30cm is not much and the area is only 27sqm in size. It hardly stands out.

That's 30m³. Where is so much water supposed to come from during a rain?

True, just checked again, it's mathematically 8 cm. Practically it's 30, because the water from the cistern overflow doesn't flow uphill.

I would bury a cistern and then use the rainwater to water the garden. If the cistern gets too full, simply distribute the excess water in the garden with a pump. This was also done here to save on rainwater fees. If everything is done by yourself, it is not that expensive depending on the size of the cistern.

: The cistern is a great idea - however, the TE still has to build the basin, as it is a requirement.

1) If I understand the calculation correctly, I have to choose between LARGE and SHALLOW (30 cm deep, 80 m²) or SMALL and DEEP (1 m deep, 25 m² wide). That is my problem. If I have an 80 m² wide depression in the lawn, it looks crooked and uneven, doesn't it?

2) A cistern only helps to a limited extent. When it is full, the water still has to infiltrate again...

The calculated values are probably based on the sealed surface; the offer mentions something about "Determined infiltration area according to DWA A138." Whatever that means