Rainwater must be obligatorily infiltrated

I have also connected all downspouts and my air heat pump to a cistern with 8 m³. That works very well for us during the summer. When the cistern is full, the excess water is diverted into a soakaway in the ground. I would do it again anytime. I buried a concrete cistern. Delivered by a Berlin shop for wastewater technology.

The thing with water prices actually makes more and more people think about the additional costs of a cistern. As I said, we actually left it out, but somehow it increasingly seems to make more sense than, for example, a carport :-/.

What do you always use as a reference? It constantly reads as if a cistern costs 10k... What capacity in liters are we even talking about here? It's probably just a triple-digit amount, you can't compare that with a carport and stuff... Get a display model from the hardware store (that's what we did) and you can save a lot. It's not existential, just some plain old cistern ;)

The cistern itself is usually not that expensive. But the installation and especially disposing of the excavation.

This is how I did it back then, right after I laid my geothermal ring trench. It has the advantage that the brine pump runs a little more efficiently. You don’t even need the shaft rings for such an insurance, but it depends on the soil. Of course, by now all of this is deep underground. I just made a note of where the shaft is under the lawn, because I didn’t want to see the cover.

Can you let the water run directly open out of the downpipe with it?

Maybe you could, but I wouldn’t recommend it. You’re introducing all the water close to your foundations, and with clayey soil, it will hardly infiltrate well. With permeable paving, it’s simply about the driveway not being sealed, so rain can still infiltrate there.

Is it really that simple? How do you get the water from the roof into it then? If it infiltrates slower than it flows off, won’t it back up in the downpipe again?

You would lay a pipe. The hole would of course have to be big enough so that water doesn’t constantly stand in the pipe. I haven’t done something like that myself (we are required to have a cistern) but I have read about it in gardening books. An alternative would simply be to set up rain barrels; that also collects normal rain without a cistern.

We don’t yet know what kind of soil it is (we were always told that a soil report is only done once the building permit is granted).

Call the municipality; they should also have a soil report from the site development. That is sufficient for the assessment. If you’re building with a basement, I would definitely find out if you have to build with a waterproof concrete structure (“white tank”) in case of clayey soil. If so, you can pretty much forget about infiltration on your property.

The excavation was not insignificant. But fortunately, we were still able to use our soil well on the property. After all, we still had to fill about 50 cm.