Rainwater infiltration

Usually, a soakaway is combined with a cistern to introduce the water with a delay. Part of the cistern is used as an "intermediate storage" to then slowly feed the water to the soakaway via a throttle.

Is that really common?
Why provide retention if the soil does not require it? There are certainly soils that allow infiltration.
From my previous experience, a cistern is significantly more expensive than depicting the corresponding volume, for example, with a larger soakaway body.

So, I have contacted the real estate company, which also had a soil survey conducted. They told me that what the other soil survey concluded is complete nonsense. Therefore, no additional rainwater infiltration system will be required. I will also get this in writing and give it to my architect. Then we will see how things proceed.

So no extra rainwater infiltration system would be needed.

And what is supposed to happen with the rainwater?

And what is supposed to happen with the rainwater?

It seeps into the ground. At least according to the appraisal by the real estate company.

so, I have contacted the real estate company, which also had a soil survey done. They told me that what the other soil survey concluded was complete nonsense. So, no additional rainwater infiltration system would be needed. I will also get this in writing and give it to my architect. Then we'll see what happens next.

What kind of real estate company is that? Do you want to build with them? Who takes responsibility if soil survey 1 is correct and infiltration does not occur? Who gave the requirement regarding infiltration? Probably the municipality. Who will discuss with them whether infiltration, as requested, is necessary or not? The municipality will always refer to their survey and insist on it. This could possibly lead to a lawsuit, in which a third survey would then be required. You should already plan for such costs, as most legal expenses insurances do not cover legal protection during construction. Even if you were to take out suitable legal protection insurance now – they won't cover the costs since the facts of the dispute were already known BEFORE the insurance was taken out.

So no additional rainwater infiltration system would be needed.

And

It infiltrates into the ground.

So either your downspouts have a 60° bend at the bottom and end open just above ground level, the water splashes out and infiltrates. Or a structural system for infiltration is constructed:
- Swale,
- Trench drain or
- Shaft.
This structural infiltration system can (but does not have to) be supplemented by a system for water usage (cistern, possibly with a domestic waterworks, if desired ...). And then it could be executed so cleverly that the water usage system simultaneously functions as retention (which then reduces the possible usable volume).

Oh yes, of course the infiltration system must be dimensioned (designed, planned); basics for this are: sealed area, rainfall amounts and frequency (i.e. region), soil characteristics.