Looking for ideas for retention volume

Hello everyone,

we are currently planning a new building on a plot of land that has been part of a floodplain since 2017 because a small river flows behind the house. According to the neighbors currently living there, this is not understandable as it has never happened. Nevertheless, we have the requirement from the Hamburg water authority to create a retention volume of 9.5 m³.

What options do we have for creating such a retention area here if we want to "hide" the entire volume under the property/later garden?

One option would be a 10,000 l retention tank - but this option would also be quite expensive.
Do you have any other ideas for implementation or have you had to solve a similar problem yourselves? I am very grateful for any feasible thoughts. As mentioned, we have to assume that we will never need the retention area. Nevertheless, we have to present a solution together with the building application.

If further information is needed, I can gladly provide more.

Thanks in advance for your feedback.

Florian

to create a retention volume of 9.5 m³

sounds to me possibly like a case for the landscaper: The overflow of a small cistern flows into a slightly excavated garden area that has been sealed by the landscape gardener with a clay layer beneath the topsoil. On 10x10 m, for example, the area would only have to be about 10 cm deep. Rainwater can then collect there and be discharged through a regulated outlet. How large is the property, would this be feasible above ground?

Maybe one could dig a little deeper and create a barbecue area/ fire pit with log steps to sit around? Of course, it always depends on the size of the property.